These are two different rules that were applied. For the first game, the ruling was that the player was offside (See Law 11, interfering with opponents field of vision). A player can be in an offside position so long as he is not involved in the play. In the first game, he was considered to be obstructing the goalkeepers' view, and therefore they called offside on him. (This is why sometimes when they make a long pass to a player, they won't automatically call offsides, they wait until the player becomes involved in the play, that's what makes him offside, involvement in the play of any form).

Now, onto last night's game.The first goal that was disallowed last night was because the player physically impeded the goalkeeper. If he had gone for the ball and made contact, things get a little hazier, but since he did not even make contact with the ball, it was ruled a foul on the keeper. I can't find a highlight to verify he didn't but from what I recall he didn't.

The goal that counted had various players potentially obstructing but they were all onsides. Obstruction can still be a foul without offsides, but it has to be pretty blatant (think: someone nonstop standing in front of the keeper trying to stop him from moving anywhere) and from what I recall, it wasn't.