From what I understand, the gate numbers correspond to the stand numbers used by pilots and ATC. At Heathrow, they use a three digit format, with the first digit indicating the terminal, hence all T5 gates begin 5xx. Gate A1 is stand 501, C55 is 555 etc. As Gate A4 is a bus gate, there is no stand 504, nor a 510 (A10). Despite there being no gates 24-27, there are stands 524-527 - these are the T5A bussing gates that can be seen from the south side of T5A. However, looking at ground charts of T5, it seems as if they decided to start stand numbers at T5B and T5C from the next group of 'tens', presumably avoid confusion, hence there is no stand 528, 529 or 530, with T5B stands (and therefore presumably gates) starting 531 (B31). Likewise, there is no stand 549 or 550, T5C stands start 551 (C51). I might be wrong, but I hope that it seems a clear and logical explanation!