Looking for flying pigs there, so many do that.
There is no exact measurement that will come that close as the part is rubber and varies, Two of the exact same tire will even be different if coming off a different set of molds and vary up to 1/4". The /80 or /90 changes it even more. That is the percentage of height the sidewall is to width of tire and it alone will change the width of tire even at the same marked width, say 110 here. Nothing weird about it. The rim width too affects the ending tire width and wildly, just it will take the same exact tire up to variance of again up to 1/2" or 12 mm. The 4.88" given there is probably using a much wider rim than you have. The bead and rim edge type (tube or tubeless) can easily change the tire width too, it easily can cover one step or more in the /XX number by the way the rubber flares out from the rim edge. I've seen 1/4" there alone.
Meaning that advertised figures of a tire size are often garbage, you have to actually fit the tire to the rim to really tell.
Measuring to the chain is often a mistake, you have to consider the swing arm front loop which is commonly the limit.