I found a typo in the code that build the mapping tables, causing multiple matches to not be inserted at all. Most of these were topics posted across several fora. When accessing these old URIs, you will presented with a list. Anyway, they were rebuilt, and I made some statistics for whoever is interested.

Universe Today topic mapping stats (matches):
T: 8530 (100%)
1: 8127 (95.3%)
2: 358 (4.2%)
3: 34 (0.4%)
4: 9 (<0.1%)
0: 7 (<0.1%)

BABB topic mapping stats (matches):
T: 20580 (100%)
1: 20498 (99.6%)
2: 72 (0.3%)
7-8: 2 (<0.1%)
0: 8 (<0.1%)

On another note, if Phil sees this, he might want to add these lines to the .htaccess file in the old BABB. It will redirect one type of request that is frequent in Google results (list a user's posts), but it doesn't solve the "user profile" links (must make some id mapping there).