This is very true. This is why nothing beats playing a lot of games with an opening and finding the variations that come out of them yourself, going over the games and finding the weak points, etc. Especially with players who don't know the theory, things go out of book quickly.
This is also why memorizing lines is probably a bad idea. A better way to understand an opening is to ask of each move-- "what does it do? why wasn't this other move better? what threat could it make to which the opponent has to respond?" etc. Especially when your opponent makes a "seemingly pointless move," very often it will in fact be mostly pointless (there's probably a goal in mind but the opponent might not get to execute it in time or you have an easy-enough response). For these cases, you should have a "null strategy" in mind, the kind of thing you plan to do over the next 5 or so moves in case your opponent doesn't do anything radical. Often this involves setting up your pieces such that they are in position to unleash an unopposable combination leading to material loss or mate.
The word "unopposable" is in there to indicate that the other player can't thwart your plans. Often it's better to keep moving pieces into position and shoring up weaknesses, eliminating counter-play before unleashing the combination. A plan that can be thwarted often just leaves you off-balance and ripe for counter-attack.
Ive noticed after studying typical openings and master games, I feel excited about using those openings. But my hearts sinks when i see my opponent deviate from the line with a seemingly stupid move, and I am at a loss for how to exploit it. Sometimes i will go into game explorer, and I cant find examples of what my opponent did, and I am unable to find how to exploit their seemingly pointless moves.
Has anyone ever had these feelings? How to fix! How to fix!?