So... the engine changes its mind?

I analyze just about every game after I play it. I'm basically trying to memorize as many engine moves as I can so that I can use them in my games.

However, there is one problem: the engine doesn't always say the same thing. 

For example, in https://www.chess.com/analysis/game/live/37029077537, the engine says that after Bc5 (move 6) and Be3 I should play Bb4.

But previously, the engine has said to take with the knight then play Qe7, which is what I played in the game. Now the engine says this move is only "good".

I looked through a lot of my recent games, hoping to find another game where this position occurred and the engine said to play Nxd4 + Qe7, but I couldn't find any games.

Has anyone else had this problem, and what is really the best move in this position?

Thanks.

The engine analyzes at different depths, that is, how many moves deep it looks at. It changes depths sporadically, I'm not sure why, but you can see the depth at the top right of the Analysis tab when you're analyzing a game. The higher the depth the more accurate the engine rates your moves.

Thanks for letting me know. That makes sense now. I was very confused.

No problem

But won't I start out the opening with at least an equal position with whoever I am playing? Or advantage against people who don't know openings?

I understand that the best way to improve is to study tactics and middlegames/endgames at my level, but my opponents seem to frequently destroy me in the opening and know many more opening moves than I do.

I see this also, but don't think it's a changing depth thing. I get the impression the engine etc. updates over time based on one thing and another, like all the games played perhaps. You see this was brilliant moves also. I.e., you can play a brilliant move, save the game to your library, then go back and look at it a month later. That move is no longer brilliant as I guess the engine caught up so to speak. But I'm just typing to avoid doing something actually useful, can't say with much confidence.

Don't just memorize engine lines for openings. Engines don't have "opening books", so you will have to look elsewhere for theory on certain openings.