Generally speaking, westbound flights coming from Europe tend to enter US airspace further north because they are typically using more northerly tracks across the Atlantic than they do when they go back eastbound. This results in 128.05 and 134.95 being quite busy from late morning to afternoon. 133.45 is not as busy at this time of day, in fact, you might hear very little on this frequency during the day.
In the evening however, when all the flights go back east across the Atlantic, they usually use more southerly tracks across the Atlantic so you would hear more activity on 133.45 because of this.
I hope this makes some sense
Sent from my SGH-M919 using Tapatalk