So we get this question all the time. Let me do some 'splaining.
Most vinyl is manufactured outside of the United States and machines are built on the metric system if not everywhere else, almost everywhere. What they put out as a master roll is usually 150cm by 50 yards. Perfect even number and easy to remember etc., but convert this to the US standard (divide by 2.54) and it comes to 59".
Now we have a 59" roll and most desktop machines use a 12" width mat. This is where the math comes in. It does not divide evenly, so we cannot cut the 59"(150cm) rolls down to an even number of 12" rolls. But we can make the math work if we cut each roll to 11.8" (11.8 x 5) and use all of the 59" master roll.
Same reason if we cut it down 4 times and sell as 15" rolls. The actual math makes it 14.75" and not 15". This is an industry standard for HTV and for ease of advertising most companies describe their product as 12" or 15" because it just looks better and easier to read than 11.8" or 14.75".
It is always a good idea to know this before designing so that you can maximize the HTV without having issues with the sheet or roll being short and causing headaches for you down the road.