Selection and Edit Mode

Hi!

I don’t know if this is the correct place to post my question (I didn’t know If I should post it here or in the Basics & Interface subsection). It’s related to something that maybe it’s simple, but I don’t know how to fix it.

For several days, every time I go in to Edit Mode and select one part of the model, randomly other parts are selected too. This doesn’t happen in the wireframe mode.

Here’s an example:

Why does this happen? Is there any way to avoid it?