Yes, differences in the bottom level of footings is common.
Single story buildings are more likely to have similar footing all around as they are supporting similar loads. As the height and weight of the building above ground increases you are more likely to see changes in the depth of the footing.
Another commenter mentioned that the depth of the footing is due to weight bearing. While this is a factor, it isn’t significant until the footing gets quite deep in comparison to the footings other dimensions. Mostly the depth of the footing is to either increase the overall mass/ weight of the footing or to keep the concrete from cracking and splitting under the weight.
MKDesign 