When set to a fixed number, is the force pool ever supposed to increase?

I'm messing around with a mission where I set my force pool to 100. After getting a message my force pool had dropped to 97, about a half hour later I received a message in the radio chat it had increased to 103.

I was under the impression that only setting force pool to "dynamic" instead of a fixed number would cause defined increases and decreases. I take it that's incorrect or have I discovered a bug?

The "Reinforcement Type" dropdown is admittedly a little confusing. The hint seems to refer only to where the insertion can take place so perhaps when that's set to dynamic it's also designed to work similarly to a "dynamic" force pool where you get bonuses for conquering objectives?