10/20 nl 3 handed calling station makes a river check raise

[mgsimpleton]

ok so villain in this hand is a calling station. a littl ewhile ago he correctly called me down with middle pair for 3 bullets. i have tightened up considerably but i doubt he has noticed... and a few hands ago he incorrectly called with middle pair for 3 bets and lost a bit of money.

For some reason the converter isn't working so I'm doing this manually.

Hero (2078)
Villain (1575)

I have A [img]/images/graemlins/spade.gif[/img] 7 [img]/images/graemlins/club.gif[/img], I make it 55 on the button. Villain calls from BB.

Flop (120): 9 [img]/images/graemlins/spade.gif[/img] 7 [img]/images/graemlins/diamond.gif[/img] 5 [img]/images/graemlins/heart.gif[/img]

Villain checks, hero bets 75, villain calls.

Turn (270): 9 [img]/images/graemlins/heart.gif[/img]

Villain checks, hero bets 175, villain calls.

River (620): 5 [img]/images/graemlins/diamond.gif[/img]

Villain checks, hero bets 400, villain lets the time expire... at this point I'm sure i've got him and he's going to call but I'll take it... then all of a sudden...

Villain is all in for 1270 total, 870 more.

What do you make of this? I know heads up is hard to read but I was so thrown off by this, I'm just looking for input.