That's an interesting point. Laptops get replaced faster, and when a laptop is replaced, it is typically left in a non-running state in which it will not experience or report errors. Desktops are often left running for months or years after they've been replaced, either through laziness or because of a desire to keep them around for a special purpose.
"we only count failures within the first 30 days of TACT (total accumulated computing time), for machines with at least 30 days of TACT."