Which computer are you playing? For example, the Easy computer on chess.com makes silly moves quite frequently, so it will inevitably deviate from the main line very quickly. As for the Sicilian line above, I can't comment too thoroughly because my strength is limited, but I don't think 4...e5 is a very good response in that situation. I'm not surprised that it doesn't show up in the database. 4...Bg7, for one, would be much better in that case.
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It is surprising how many potential games are not in the Game Explorer! For instance 1.e4 c5 2. Nf3 g6 3.d4 cxd6 4. Nxd4 e5 results in a "Not in the database! You would think that some master, sometime, had played this potential game.
Some people study some lines, like the Ruy Lopez, to death, going 20 moves deep or so. To me, at least, this is like a "I do this, you do that," very choreographed sort of way to play. I guess this is how people play blitz chess, lots of pre-determined moves.
When you play the computer, at least the apps I have encountered, it deviates very quickly from the standard mainlines you read in books or see in masters games. Given this, do you think it makes sense to memorize mainlines?