I don't think it's too overly complicated to explain (even though the system itself has several variables). Posts above mine have all the info on HOW you get upgraded.

I fly EWR-BUF, EWR-FLL all the time and get upgraded ~70% of the time as a 1K. When I don't, it's pretty clear that folks have paid for F (or used an instrument) - whether it's a group of foreign tourists heading to the Falls, or some wealthy New Yorkers heading to their 2nd home in SoFL, I'm usually knocked out because of paid F. Days of week / time of day are totally a factor, too.

With things like IAD-SFO and hub to hub, it's totally conceivable that folks are buying F outright (with OPM or otherwise). That's the gamble we take - CPU's are a 'chance' at an upgrade.

Re: GPU's, do a search and use Expert Mode to your advantage. Be flexible. I almost always apply GPU's at time of booking in order to instantly confirm. Anything else is a gamble, and potentially a pricey one.

It certainly doesn't make me hate United if I don't get upgraded; it just means I missed out. Grab the exit row, a drink chit, and enjoy the trip regardless - Y+ isn't horrible, and it's quite amazing that UA get you from A to B quickly (usually) and safely for typically a relatively small amount of money!