Temporary invulnerability?
 

I find that when I hit someone from point blank range, they don't get hurt. For example, I was attacking a guy who was blocking a mining rock (yeah, well I should have :( ) and the first time I hit him, he was hurt. The second time, he was hurt. So was the third and fourth time. Basically every time I hit him, he was hurt. However, when I opened my RPG window, I discovered that only my first two hits counted as "blows" and the other ones were misses, despite him getting hurt and attacking from point blank. What's going on with this?

At low weapon proficiencies, you'll miss at point blank range.

It's also possible you were slightly past him and were actually swinging at a guy one pixel behind you.