they are two separate data streams going through the net.

Each has it's own path, and one is longer than the other.

If you restart one of the streams, I'll bet you see a different lag time.

I feel a little like that politician who thought the internet was a series of tubes.

The exact path is determined at the time of connection based on up-to-the-millisecond traffic loads, and as such, is never the same twice.

Sen. Stevens was a total jerk, but "tubes" is not a terribly inaccurate metaphor. Concepts such as bandwidth, throughput, and load balancing apply.

Your pc probably has a regular hard drive. Your tablet's OS is likely optimized for that and probably buffers less data before it starts playing. That along with what earthshine said about start times may explain most of the difference.

not that I have much of clue about solid states or hard drive physiology - start drifting into electricity and I get a big old mental block. seemed strange to me to be that much different.

has more to do with that then the drive, although a ssd can help. Ill take more/faster ram and a regular drive than less ram and a ssd anytime. Video loads into ram, not the hard drive for streaming purposes.

can be much stronger than tablets, more memory, faster wifi, and better processor just because they have more space to utilize. If you have a pretty good desktop then that would be the reason.