Believe as you will, since most people do, but that's not the reason. As I said earlier, the powers that be at BFD wanted everyone to get exposure to medical calls. It started out with ladder trucks doing medical calls on Saturdays only, "Saturday is Ladder Day!". Then it expanded to alternate months as it is now.
The other factor was that senior men were transferring to ladder trucks and other units that didn't do medical calls since those units weren't running their butts off all night long. So, a sort of load balancing was put into effect. One month ladders (and the Rescues) would go, the next Engines would go. August is an Engine month, in case anyone is really interested.
Also, if the first due Engine is on a call, now the ladder is sent instead of the second due Engine.
For those who are really interested, Engine 21 in Edward Everett Square does the most medical calls. At one point they were called "Ambulance 21", but since Boston EMS has one now (on some days), they are affectionately called "Engine 321".