Different Deletion from seemingly equivalent motions

Question

I have been learning vim for some time now and I am loving it but there is something that confuses me a little bit.

For discussion proposes lets say that we have the following text foo.bar with the cursor over the f. If I press w the cursor then moves to the dot .. Alternatively if I press f. the cursor moves to the dot as well. So it seems that both movements are equivalent in this case, so I would expect that if I use d {motion} the result should be the same with either motion, but in the first case the result is

.bar

and in the second is

bar

so they are not really equivalent. This breaks my initial impression that we can think modularly about the commands, like thinking independently about actions and motions.

I apologize if this is too basic.