"but are PC TV tuner cards so very different from a freeview box?"
Is it a digital terrestrial card, or a digital satellite card?
No they're not so very different, but as between different makes of freeview boxes, they may have different quality (ie less sensitive) tuners.
Analogue reception and digital reception is so different that you really can't compare them. You must have a wideband aerial (and wideband amplifiers and splitters where required) - your analogue aerial may be for totally the wrong frequency and, depending on where you live.
Are you giving it the same quality input signal when you compare one with the other? Every time you split the signal from the aerial, you reduce the signal power cconsiderably