Streaming works by keeping a few seconds of video stored ahead of what you are watching. Buffering happens when that reserve empties faster than it refills, which is a consistency problem rather than a speed problem.
A connection can average high speed and still deliver in bursts. Brief gaps — a neighbour's microwave, a crowded channel, a router restarting a link — drain the reserve, and playback pauses even though a speed test looks fine.
Streaming services also adapt quality on the fly. Before stopping altogether they drop resolution, which is why a film can look soft for a minute and then sharpen again.
Because televisions often sit far from the router, and many use only the slower band, the TV is frequently the weakest device on an otherwise healthy network.