The surface scanner is really buggy. I use one, it tells me that there is oil x blocks away in a direction, I place on a few blocks further in that direction and that one points in a completely different direction and/or gives a completely different range. This makes it kinda impossible to find oil.
Here is an example:

Same issue, basically useless at finding oil.

I noticed the same thing, I assume it's switching between different deposits instead of staying with the closest one. But eventually I found one, not too unreasonably far from where I expected... Well, I had vein miner at bedrock, so maybe it was a bit far.