It really comes down to a law of averages, no matter how you clean, during the course of a day there will be so kind of a blemish, spot, smear, run, drip, etc etc etc, you just cant get perfection on every single pane of glass on every single house, let alone frames and sills aswell,
how many of you have cleaned a house regularly to one day find dirt underneath the sill that you didn't think was there, but somehow its been tucked up in a spiders web for months, and only now dropped down, or gone back to a job found spotless window, and just a few slight dirty water marks on the frame,
or you didn't quite wipe the edge of the seal, when you was using the cloth,
i did a house on Wednesday, patio doors and window were caked in sand etc, so sprayed with some tfr, scrubbed and rinsed, 10mins later came back scrubbed rinsed again, went back 4 hours later, they were perfect, but the window at the front, that was reasonable after 6 weeks still had a few little spots
so after all that ramble law of averages