I suspect most of us book “Both Parks” even if we are not sure we are going to hop, because there is no downside to doing that (although we ALWAYS hop, so I really do always know we need ”both parks”
). I think DLR is limiting reservations for DL more than reservations for DCA, due to expected high crowds at DL for ROTR. That is there are only so many “both parks” reservations available, which are limited by DL availability. Once those run out, there are still “DCA only” reservations available. This is my theory... This pass was likely designed to spread out crowds, among other priorities for DLR. Limiting guests from DL reservations when DL is going to be crazy crowded will effectively do that.