If you noticed a gap between an RMR HD, SRO, or RCR, it can be one of two causes:
1. The sight's integrated sealing plate is not 100% flat (it's been seen more often)
2. The plate itself isn't 100% flat (observed, but rare)
In both cases, this is cosmetic only, the gap doesn't any effect on the plate or sight. As long as the plate can hold the sight and keep it immobile, the gap is a non-issue.