Had a listen on YouTube and it is a different mix. Never noticed that before. Sounds like the string parts are all on synth instead of live instruments. Maybe it was a mistake and an earlier mix was used, or it could be deliberate, maybe they'd decided not to have people on stage miming the violin/cello parts... That sounds like an odd decision to make, but Top of the Pops often had weird rules like that, probably to do with licenses for the recordings and/or additional musicians used. Back in the 1970s bands were allowed to mime, but sometimes it's clear they weren't allowed to mime to the actual single, and they'd recorded a new version especially for Top of the Pops and then mimed to that instead! One example is Starman by David Bowie, and another is Good Old Fashioned Lover Boy by Queen.