I am not an expert on slavery in British colonies, so I am going to wrestle with the second part of your question, which is excellent but also extremely difficult to to answer. First off, I must mention that population figures from the colonial period are notoriously inaccurate. Censuses butt up against challenging questions (at the time) of who counted as people. How much effort should colonists expel to count populations that were not always considered fully human? This led to varying population figures. So if the chart you are referencing is a primary source, these numbers most likely are not precise because they would reflect the limited abilities of people at the time to count fluctuating populations of non-white people often far from population centers. If they are modern estimates, again, take them with a grain of salt. These numbers can vary widely because primary sources are so challenging to rectify and synthesize. From populations figures, we can learn generally the make-up of society, but precision would come much later.

Secondly, populations were always changing. If you were looking at a chart of free people of color in the 1600s, you would see a much larger proportion of non-free slaves to free people of color in Spanish colonies. Likewise, the population of Barbados would be significantly different because the island’s primary labor source originally were indentured servants. African slavery came later. So the population figures are general snapshots of society at a particular moment in time.

That being said, what accounts for such a difference in the population figures? Many factors! Most important is the indigenous populations of these islands, who were considered in Spanish societies free people of color. Barbados’s indigenous population by the early 1600s was virtually non-existent. Although there had been large numbers of indigenous people, they had mostly been killed by European diseases or carried off by slavers. The first British inhabitants of the island considered it uninhabited, so they needed to look elsewhere for labor, which eventually led to a clear dichotomy between white and black. Labor needs were first filled with indentured servants, but plantation owners increasingly turned to imported African slaves who were considered less unruly.

Barbados is somewhat exceptional; labor relations in the Caribbean under Spanish rule were quite different. Here, indigenous populations, though devastated by disease and Spanish labor requirements, did not completely die out and eventually rebounded. Even with the introduction of sugar, indigenous people were cheaper to mobilize than imported Africans (although still considered weaker workers). Therefore, it took Spaniards longer to turn to Africans as a significant source of labor. With larger populations of people of indigenous descent, indigenous people remained a prevalent part of the population. Over time, this would lead to very different racial dynamics in Spanish society. Throughout the Spanish Empire, there was a huge swath of people of color who were not necessarily African. These castas included mestizos, pardos, mulatoes, and zambos. By the late colonial period, Spanish racial systems had become so complicated that these terms were virtually useless. In fact, people of color could buy certificates that said they were legally a different race. It gave a further degree of flexibility to the Spanish system. Only creoles and peninsulares were considered white; everyone else fell into the category of free people of color. Lastly, in many Spanish Caribbean colonies (such as Puerto Rico), sugar economies did not take hold until much later, which led to smaller numbers of Africans being imported.

Although the idea of “the black legend” is prominent when describing Spanish involvement in the New World, slaves actually had more flexibility in Spanish territory than on British plantations. Spanish slave owners frequently freed their slaves upon their death, their slaves who provided long and loyal service, and their children they had with slaves (all of which happened more frequently in Spanish America than British America). African slaves were able to save money and buy their own freedom more often than slaves in Barbados, which was the primary way that slaves gained their freedom in Spanish colonies. Additionally, status of free or not free passed down on the mother’s side, so if a male slave and an indigenous or free black had a child, that child was considered free (although subject to the respective labor demands made on indigenous people for example). Finally, Spain had stronger laws protecting the rights of slaves. They had the right to marry whom they wanted and the right to hold some property. Many slaves successfully used Spanish legal systems to their advantage to gain freedom.

Don’t get me wrong, both slavery and encomienda in Spanish America were cruel and inhumane labor systems that led to the deaths and coercion of tens of millions of people. Unfortunately for Africans, being sent to the Caribbean was often a protracted death sentence, especially where sugar economies dominated. But the differences that I have noted, spaced out over centuries, created the unique populations about which you are reading. I would recommend checking out J.H. Elliot’s Empires of the Atlantic World for an well-written comparison between Spanish and British empires. Also Burkholder and Johnson’s Colonial Latin America and Higman’s A Concise History of the Caribbean are excellent introductory texts.