Young rower with questions

I’m 15M 6’0 162lbs and I want to row in college. My 2nd ever and most recent 2k test was a 7:24(1:51/500m) about 6 months ago after pulling a 7:39(1:55/500m) the week before. I’ve done 3 months of rowing on the water and 3 months of consistent erging since then. Despite all of this rowing, in a recent 2x1000m workout my splits were 1:46 and 1:50, which was underwhelming. I was projected to be able to hang on to a 1:46 with the knowledge that in a 5x5’ workout @R20 I averaged splits of 1:54, showing that I have enough leg strength to do so. I’ve been doing a lot of steady state so I think my endurance should be up to speed with my strength. I feel like I am simply inefficient at high rates while my lower rates are seeing massive improvement. Why is this? Any tips?