The cloud of an airburst of the height and yield of the Hiroshima and Nagasaki bombings is really several distinct parts. At the top is the "bomb debris cap" — this is the remains of the actual nuclear fireball that has cooled and is rising through the air. Below it is a "bomb debris stem," which is remains of the fireball which has cooled a bit and been pushed below the main cloud.

Separated by a substantial gap in the case of the Hiroshima photo is the "dust stem" and "dust base" — this is material disturbed by the explosion and smoke from fires and so on that has been pulled up due to negative pressure. It is not made up of the nuclear fireball residues and is not that radioactive by comparison.

In the case of the Hiroshima cloud, you can distinctly see the "bomb debris" cap and stem at top, and a gap, and then the "dust stem" and "dust base" below. They are two different parts of the cloud, and the wind has blown the top part just a little off from the bottom (winds can vary quite a bit at altitudes of these heights). Note that this was taken pretty quickly after the bomb went off; the clouds will have stabilized, merged together somewhat, and become more of a column in a few minutes.

Note that you can see the distinct separation very clearly in the early Nagasaki cloud as well. There, as in Hiroshima, the two clouds are slightly off-axis, probably again because of different wind at different altitudes.

This kind of effect is the result of the scaled height of burst (which is to say, the height of burst relative to the yield). Things like the angle of the bomb in the explosion do not affect the cloud.

For more technical details on cloud terms, etc., see Spriggs et al., "Fallout Cloud Regimes," Countering WMD 21 (Summer/Fall 2020).