Why is isolated car battery losing charge so quickly?

Battery had been left no start for months until I found it at 0.45 or so.

Used a charger after an entire day it got to 2.95 volts.

Then connected jump cables to the isolated battery and revved it bringing it to 11.66 volts in 8 minutes.

Left isolated battery in boot and it’s down to 10.5 volts. Can’t be a parasitic drain because it isn’t connected. Would have got colder I’m thisd two hours but I don’t think that causes voltage loss.

So what does all of this tell you about the battery, why did it lose more than a volt in 2 hours even though not connected?

Thanks