I am a bit perplexed understanding the meaning of the values of MON$STATEMENTS.MON$STATE.

Here is what I found in the FB 2.5 Release notes:

Memory Usage in MON$STATEMENTS and MON$STATE

Memory usage statistics in MON$STATEMENTS and MON$STATE represent actual CPU consumption.

Tracker reference: CORE-1583)

The tracker referenced here is not clear on how the MON$STATE represents CPU usage.

<SL> MON$State *does not* reflect CPU consumption, it only reflect the operational state of the statement.  The tracker details should be updated to reflect the true nature of the change made.