That is because the ballasts are not the HO part. The lamps are. There are two things you have to look at on a ballast: The wattage, and the lamp type they are intended to run. This is because the lamp specifications came before the ballasts.
T5/HO lamp wattages are 24w, 39w, 54w, and 80w. The lamp types are F24T5/HO, F39T5/HO, F54T5/HO, and F80T5/HO. There is a fifth HO lamp, the F49T5/HO, but that is very uncommon in this hobby. For an additional example, a F96T12/VHO lamp such as the T12 VHO Super Actinic that can be run on an IceCap 660, that most are familiar with. (Good lamp incidentally.)
The third thing to look for is the start type. T5/HO uses a programmed start, not an instant start, or rapid start. They are different, and running a T5/HO lamp on the later two can cause problems.
The ballast catalog numbers will usually have the lamp wattage, and number of lamps the ballast will run, but it can be a chore, because there are no U.S. standards, as there are for some Metal Halide lamps. M80 for instance for a type of pulse start MH lamp. This is because the lamps are European in origin. Also the T5 lamps are metric, not U.S. Customary, so a 48" 54watt T5 is actually 45.8" - 46.something" inches, for instance, for those that want to build their own fixtures.
Generally the catalog numbers will cross-reference however. Advance numbers are ICN-2S24-T, ICN-2S39-T, ICN-2S54-T, ICN-1S80-T. You may run across a catalog number such as ICN-2S54-90C-T, but don't let that throw you, the 90C is just the maximum start temp. (standard is 70°C)
There are a number of companies making ballasts that meet the T5/HO specs. A couple are marketing ballasts, and assembling fixtures, for T5/HO that do not meet the required specs. The Fulham Workhorse and IceCap 440 & 660, being the biggest offenders. However, if you don't mind using the most unreliable ballasts on the market, the Fulham Racehorse is a correct T5/HO ballast.
https://www.1000bulbs.com/category/t5-fluorescent-ballasts/
