Hello and welcome to the board.
If you search on this it has been discussed before. The reason for this is that the hour meter counts time based on actual RPM vs. rated RPM. Therefore, if your tractor's rated speed is 2,600 rpm and you run it at 1,300 rpm then it would take 2 actual hours to have 1 hour on your hour meter. Likewise, if you run it over rated RPM then time on the meter will accumulate faster that real time.