This might explain a bug that's been bugging me for years: text selection in preview tends to fail when working with PDFs printed from Firefox (and others). The selection jumps from the cursor and includes too much text from below. If I select from below it jumps and selects from above. Not just words, but sometimes entire extra sentences or paragraphs.

This is quite similar to iOS text selection in practice: mostly works, but when it doesn't you're screwed.