You didn't get all of the oil out in the first place.
The sump is large flat shallow shape so a slight slope has a big difference. It is around 10" x 12" x 1.75"
And generally you put the mower on a nice flat floor, but if the front left tyre is lower pressure than the right you will not get it all out.
In the shop I run the left tyres on a piece of 2x4, pump the oil out go do something else for 10 minutes come & pump again the repeat a third time.