It doesn't get much simpler than this. Deviation and frequency steps are two totally different things. FM/FMN deviation means how wide or narrow the modulated signal is, or how much bandwidth it occupies.
Frequency (or tuning) steps (2.5, 5, 10, 15, etc) is the increment that the radio tunes when you tune the VFO dial. If you have 5 kHz steps and your VFO is set at 145.000 MHz, when you turn the dial it will go to 145.005, then 145.010, etc. If you had 20 kHz steps then tuning dial would change from 145.000 MHz to 145.020, then 145.040, etc.