Safari downloads sometimes continue, sometimes restart I'll be in the middle of a long download (a software package or movie) and a network glitch will halt the download ("The network connection was lost").

In the Downloads icon on the Toolbar, I see the file(s) I was downloading with buttons beside them that have little orange circles to indicate "Resume." But sometimes the button continues the download and sometime it restarts the download from scratch.

Does anybody know why this happens and whether I can tell in advance which will happen? Also, the gray x "Stop downloading" button usually restarts the download from scratch, but if most of the download is complete, it might just suspend the download and allow me to continue it. Does anybody know the threshold for this behavior? Half-finished, perhaps?
This is a function of the server - not the client, so there is no download threshold.  Check this answer for a bit more information on how to check whether the server supports resuming http downloads.