hard doubt it's bc of old data. It could be a bug in the system, but most probably It's either bc you uploaded from another device/browser or bc of gmail or after the free period ended

But compressed images were free forever until they weren't, right?

nope, it was unlimited compressed for a specified timeframe*(for other phones) and free unlimited compressed forever for pixel 2 to pixel 5(and unlimited uncompressed for og pixel). Google didn't break their promises * Actually it wasn't about timeframe, google afaik didn't say that other smartphones will get unlimited compressed forever, just that it's free for that time

English is not my first language, but I am good enough that I still think I was justified to be (slightly) surprised when they started charging.