I think it may be the cooking process. I just tried some standard Tesco Hot Horseradish, just a bit on the end of a teaspoon and it was hot and radishy
I dumped a heaped teaspoon on the microwave plate and gave it 30 secs on full. Then tasted it, there was only a touch of fire and it tasted very bland, the flavour had all but gone...
Dave