The reason these are just 'ballpark' information is due to the different 'key rates' reported in the client.

There are actually 3 different 'key rates'.

The rate reported by the -bench test. (this should be close to your unit completion time). This is a good estimate of your GPU throughput and is used by the client to select which 'core' (the fastest we hope..) to use.

The 'key rate' completion time you see listed when each unit is completed.
and...

The 'key rate average' that reports your running 'average' completion rate. This is usually lower than your actual completion rate.