That level of granularity should be glossed over. Having two level X monsters that are absolutely identical except for +/-3 hp or 1 attack bonus should be avoided in general, but it's even more important that the system not try to distinguish between the two in terms of experience granted for overcoming them. Rather, all level X enemies should grant Y xp, regardless of how individual monsters of that level vary (regardless of minions, solos and so on).
Otherwise you back yourself into a corner. Orcs are worth more than gnolls because they have 10 extra hp but are otherwise identical? Leaving aside the question of why you bothered making two separate monsters that are so similar, giving them different xp values opens the floodgates. What about two monsters that are identical except one has 3 more hp, but the other deals d8 instead of d6 damage? What about the two monsters that are the same, but one has +1 AC and the other can move 10 feet per round faster?
Giving monsters levels lets you roughly gauge how challenging they are, and assigns an xp standard for that level of challenge. Trying to zoom in and award xp per hp and attack bonus is clunky and overcomplicated, IMO.