Any digital services tend to introduce slight delays, so it come from that. Whereas your 'phone is self-contained, with an internal clock that once set to standard time, stays there.
Something like this was noted in the UK with the introduction of so-called "Digital Audio Broadcasts" (of radio programmes, needing an appropriate receiver). Although promulgated by wireless not cable, things like the Greenwich Time Signal "pips" were often received slightly late thanks to too much electronics between pip-generation and loudspeaker.