This add-on does not show real per-tab memory usage. To "estimate" the memory usage, each tab starts at a fixed 50 MB, then adds hard-coded amounts based on the URL (+100 MB for youtube.com, +80 MB for facebook.com, +60 MB for google.com, +70 MB for docs.google.com), plus a random 0–50 MB "to simulate real memory usage." It pretends to show memory usage based on real browser metrics but only uses a simple heuristic.