Explaining a branch with visibly different leaves on Citrus japonica
What you are seeing here is an example of a winged petiole. The petiole is the leaf stalk and these can have wings called stipules. 

According to this source kumquat leaves have narrow petioles. I can't explain why you are seeing variation on a single plant, but I recall seeing similar variation on garden citrus in California, so I suspect it isn't unusual.