size of seeds in a variety

[charline]

is the size of the seeds in one variety almost always about the same or can it vary a lot?
I am very careful for saving seeds. I take only average size fruits, not the smallest and not the biggest. And I label it very carefully.
But this time I had in a variety in one fruit big seeds and in another fruit much smaller seeds. It made me really think it is not the same variety.
can that happen? or did I really mix up something?