There are several B fares published on the route:
Export from Wandering Aramean Travel Tools
Code:
Flexible Fare Search -- LGA-SJC roundtrp for date range Dec-Jun on carrier CO -- Continental Airlines
Carrier|From |To |FareBasis |Fare |Min |Max |Adv. Purch|Earliest |Latest |
CO |LGA |SJC |BA0KY |USD$ 838 | | |seerules | | |
CO |LGA |SJC |BA0HN |USD$ 898 | | |seerules | | |
CO |LGA |SJC |BA0HY |USD$ 1244 | | |seerules | | |
CO |LGA |SJC |B1 |USD$ 1627 | | |seerules | | |
CO |LGA |SJC |BA7UPN |USD$ 1752 | | |7days | | |
CO |LGA |SJC |BCOUP1 |USD$ 1892 | | |seerules | | |
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The last two are insta-upgrades for any customer but otherwise they simply have different fare rules. That said, I do not see a $637 r/t B fare listed there. Are you sure that the $637 is B in both directions and not B one way and something lower the other?