Thanks for the answer. Acutally it's more complicated than that, because time periods depend more or less exponentially on the 6-bit rate values. And because for higher rate values, the time periods decrease.
The similar chip YM2413's manual gives a table:
The time is exactly divided by 2 every time RM increases, but it's difficult to devise a precise formula, and in any case I'm not even sure that during this period the volume decreases/increases linearly.
This could look like a detail that the ear wouldn't discriminate, but it's not because of the nature of FM synthesis (the resulting sound is not heard directly but is used to modulate another sinusoid). Very small time differences make big timbre differences in the final sound.