Nothing very unusual in that. A lot of benchrest shooters have reported accuracy to be best at near maximum loads for as long as I've been reading about handloading. Though I'll also point out you may well have missed another barrel timing sweet spot by not starting at 10% below maximum pressure. Western Powder's FAQ cites that, in addition to safety, as a reason for not starting in the middle of a published load range, but going all the way to the bottom. On the safety side it's because a lot of loads are still measured in copper crushers and, as SAAMI's example of sending the same lot of reference ammunition to 9 different test facilities shows, below, those instruments can be off by around 25% sometimes.
Due to dimensions in your individual chamber, your top load may well be above SAAMI MAP. As long as the accuracy is tight, it's not likely to be outside its own individual limits. I would suggest, however, that 0.5 grains is a little large and you might miss some accuracy sweet spots. Try about .3 grains as you go up any further.