The processor can't really add a float to an integer. It can add a float to a float or an integer to an integer. So when you add a REAL to a DINT the processor will first converted the DINT to a Real in the processor, then perform real addition, and finally convert the result to a DINT and save it to the DINT tag. Hence you still have the 224-1 (16,777,215) resolution limit for counting by ones.
This limitation is not unique to PLCs. Even 64 bit machines ultimately have a break down in resolution with real numbers.
Edit to add:
There are a number technote about this limitation in the Rockwell Knowledge Base. Refer to documents 7765, 59372, 29562, 18209, 29562.