I think i can help a little bit to understand the behavior of the check.
The “fortigate_memory_base” is interpreting all parameter values as percent.
That’s it 
You have with the actual fortigate check no change to specify some free levels with MB.
@Constey - your 82 / 85 MB are interpreted as 82 / 85 % then also the values in your graph are right.
This is not a bug - only the check cannot handle other parameters than percent.