After White's sixth move 6. Bd3 in the Milner-Barry gambit line, White hasn't yet truly offered his pawn up for sacrifice yet. It's for this reason that Black will play cxd4, but not continue the exchange following cxd4, instead fortifying against a possible Bb5+ with Bd7. What if Black chooses not to begin the exchange until he can be sure to win it? After all, is usually happy to return dxc5 with Bxc5, so there shouldn't be any rush to trade off the c-pawn. Yet when I follow the "best play" by Stockfish (and uniquely best, if it is to be trusted) following 6. ... Bd7 and arrive at the following position:
[FEN "r3k1nr/pp1b1ppp/1qn1p3/2bpP3/8/2PB1N2/PP3PPP/RNBQ1RK1 b kq - 1 8"] 1. e4 e6 2. d4 d5 3. e5 c5 4. c3 Nc6 5. Nf3 Qb6 6. Bd3 Bd7 7. dxc5 Bxc5 8. O-O
I see White has traded away one center pawn, and the other center pawn needs to be defended by pieces for the foreseeable future, and he doesn't have the attacking chances that could be expected from the mainline. Following Black's likely 8. ... a5, White can't even chase away the Black bishop with b4. Despite this, Stockfish rates White at about 0 to +0.5. What resources does he have?
There is also the following similar line, where if I didn't know any better, I'd say White ends up with many of the same problems including a meddlesome knight on his kingside, yet Stockfish rates this as about equal:
[FEN "r1b1k2r/pp3ppp/1qn1p3/2bpP3/6n1/2PB1N2/PP2QPPP/RNB2RK1 b kq - 3 9"] 1. e4 e6 2. d4 d5 3. e5 c5 4. c3 Nc6 5. Nf3 Qb6 6. Bd3 Nh6 7. dxc5 Bxc5 8. O-O Ng4 9. Qe2