I would say it is because the DLink DWA-552 adapter doesn't support the 5 GHz band.
Looking at the specs for the adapter it says it supports IEEE 802.11n and IEEE 802.11g which is the first clue. Usually wireless devices which support the 5 GHz band also include support for 802.11a. Since this adapter does not, it calls into question whether it supports 5 GHz.
Then I noticed the next line or so down in the specs is
Not all 802.11n adapters out there support both the 2.4 and 5 GHz bands. This looks to be one of them. It's one way to cut manufacturing costs, I suppose.
BTW, I don't understand the reference to you DLINK-615. The other two routers you mentioned are both simultaneous dual band routers. But the DLINK-615 only supports only the 2.4 GHz band. Right?