HINT: The letters I gave were grouped in a particular way, using a particular criterion. If I had used a different font…one with serifs, let’s say…the groupings would have been totally different…
Here are the groupings once again…
DO
B
PQ
CGIJLMNSUVWZ
AR
EFTY
HKX
ANSWER: Notice that one of the keywords I gave was “topology”. That’s it. The letters are grouped based on topological similarity.
Many Worlds Theory 
OK, I’m feeling quite stupid right now. You’ve given the answer but i still don’t get it 😦
Imagine making letters out of string. D and O are just a single loop; B is a double loop. C, G, I, J and so on are all just a single piece of string…you can bend any of them to make any of the others, without tying any knots. But H, K, and X are very different: each has four “ends”, connected in the middle…
Oh! That is fun. Though I don’t think anyone without a science degree (in fact, some very specific science degrees) will get it. Still, I had a ball… or anything topologically equivalent.