Only 100 days on my FMM card instead of 180!!

When I went thru immigration in Cancun, Mexico I was unaware that the immigration agent can write what ever the amount of time to allow you to stay in Mexico, not exceeding 180 days.

I did not realize what that I guessed 100 written across my FMM meant I had 100 days not the usual 180 days. Fir that matter I had no idea what an FMM was. What it is