I was wondering where the errored seconds per hour was picked up from.
It's calculated from the differences in the total ES values reported by the modem. The values shown on the status bar at the bottom should correspond with the values shown in the Errors --> Averages tab. On that tab you'll see the specific (up to) 24 hour period to which it applies.
DSLstats also maintains a history, which is saved when you close down the program, and reloaded when you start it up.
There are some quirks in the calculations of the averages, and sometimes this results in incorrect figures being displayed. This usually corrects itself over a period, or you can hit the reset button to start again from scratch.