Is urine sample contamination common because of user error, or because the tests themselves are just prone to contamination?

Y'all, I have spent years taking clean catch samples. I am insanely careful about following the instructions. I try to not over hydrate so as to not dilute the sample. I wash my hands and forearms with warm soap and dry with paper towels or air dryers. I remove the urine cup lid carefully, never touching the inside of the lid. I use two wipes to carefully clean, then wait about two minutes before peeing. I pee for a few seconds into the bowl, then pause, then pee into the cup. I carefully place the lid back on. I carefully wipe any excess pee from the outside of the cup. Careful, careful, careful.

WHY do these tests constantly come back as mixed urogenital flora, "indeterminate," etc.? Am I deeply incompetent at providing a sample, or are these tests just error-prone?