Specifically, JFK,EWR-BRU,AMS. Duration of stay seems to be correlated with the cheaper fares (same ticket class though). What's going on here?

Note: The following default URL dates might need to be fiddled with.

Weekend trip, 1-5 nights. Although it shows ~$1029, if you click through it prices ~$790.

Weeklong trip, 6+ nights. $460-480

I can understand that these things happen, please let me know if that's the case, I'm just hoping that this can be explained.