It's a matter of sheer scale and numbers.

Like /u/DuxBelisarius pointed out, the German bomber offensive was miniscule compared to the combined Allied bomber offensives. Britain was bombing Germany by night, and the Americans were bombing by day. Furthermore, the sheer numbers in their formations were much higher. Even early in the American offensive, an American bomber stream could include roughly 200-300 bombers. The British were able to mount a 1000-bomber raid even before that.

There was nearly 3.2 billion pounds of bombs dropped on Germany during the war. So if 5 bombs out of every 1000 were a dud, then 200,000 lbs/year would be what you get in your questions. In a wartime economy where they're mass producing these, that's not a totally unreasonable number.

Also, I imagine a significant portion of that number also comes from artillery shells as well.

The Germans simply never managed top drop the kind of tonnage of bombs that the Allies did. The German bomber offensive against the UK lasted from summer 1940 until spring 1941, and German bomber aircraft carried nowhere near the kind of payload that B-17s, B-24s, Lancasters and Halifaxes could. The Allied strategic bombing offensive really began in 1942, and continued all the way until the end. More bombs were dropped more often, hence the greater amount of duds. That said, German unexploded ordnance can be found across Europe, and it was and still is a problem in post-war Britain.

Was the article solely focused on munitions from the Second World War?

Millions of shells were fired during the First World War and most of the major battle fields were saturated by both exploded munitions (shrapnel, bits of lead, explosive and gas residue) and unexploded munitions (shells, bombs, grenades, rockets, etc).

Prior to the first day of the Battle of the Somme, for example, British artillery fired a million shells over a seven day period into a frontage of 16 miles. On the first day of the battle alone British artillery fired nearly 250,000 shells.

If you have ever driven through rural areas either in the United States or Europe, you'll no doubt have noticed all of the piled stoned walls surrounding farm fields. Stones rise to surface over time, farmers churn them up with plows and then pile them up. The same thing happens with unexploded munitions. In Northern France this known as the yearly "Iron Harvest," and it still produces piles of munitions year after year.

In many cases, the rush to produce shells during the war years to meet heavy demands meant that quality control was poor. The result was lots of ordinance that did not perform as anticipated. In addition, at least on the Somme, the chalk soils and churned up ground contributed to shells simply not exploding when they hit the ground. This problem was later partially resolved with proximity fusing (as opposed to contact fusing) that allowed shells to detonate above ground level.

Just a little to add to the fine answers here- it's important to realize that bombs are they are designed to be inert. Bombs are often stockpiled in large ammunition dumps and if one accidentally exploded, the results would not be pretty. Normally, armorers kept bombs separate from their fuses until the mission. There were multiple types of fuses as well. One type that has proven really dangerous for postwar ordnance removal is the delayed action fuse, which was timed to go off several hours after contact with the ground to play havoc with recovery efforts. If somehow the bomb fuse was faulty or damaged or hit at an odd angle, it might not go off.