There are times when your client is showing a different block than what you are actually doing, so that would explain being hit through blocks. If you are actually lagging, players become much harder to hit, so it is likely the attack or shot you thought would miss actually would have, if it was not actually lagging.
There are some quirks that seem like lag that come down to weapon's hitbox. For example, all thrusts can be turned beyond their physical representation and hit, two handers especially have an interesting stab that is dangerous at close range, and overheads have lingering hit detection that will still hit if a player drops his or her block when the other play turns their already swung weapon into the character.
