I think you've misunderstood, usually you're not meant to explain jokes but I'll act in good faith and do it anyway.

The comic uses 9"5' which is obviously meant to mean 9 feet and 5 inches.

But in actual notation ' is for feet with " for inches. Meaning that what is in the comic actually reads as 9 inches and 5 feet if taken literally.

So the way it should've been written for it to mean what it was meant to is 9'5".