Comparing a Panama Canal cruise in Dec,. and a Bermuda cruise in July are two different animals...one is off peak season, one is during peak season.....completely different itineraries.... You cannot compare the two at all.
TA's that are charging a higher fee are doing so for one of two reasons:
1. Charging a fee for their service
2. TA is not researching for the lowest fare you may be eligible for.
Some TA's may have a lower fare due to Group Space they have secured quite some time ago and those fares are lower than the current cruise fares.
Some TA's will rebate some of their commission to offer a lower fare, but not all cruiselines allow this any longer.
Because of the limited window for Bermuda sailings and minimal ships headed there, the cruise fares have historically commanded a higher price, especially during July - Sept.