WHY???? Midrange magnet's vented pole piece sealed by design - Thiel CS3.5 ????

[dtm1962]

I have a pair of Thiel CS3.5 3-Way speakers that have Scanspeak 4-1/2" midranges (13M8621) used in a sealed enclosure. The magnet's
pole piece is vented to allow no buildup of pressure behind the non-porous (coated) center 38mm dome to vent when there is any excursion as a result of higher SPL's.

Question: Why would the pole piece vent is covered to seal in the back of the center dome to allow a scall subenclosure behind dome but the cone is allowed to vent to a 2 liter midrange enclosure?????

Why would Jim Thiel design this way. I would think it would cause some non-linearities/harmonic distortion as a result of the cone vents to a 2 liter enclosure while the center dome vents to a .001 liter???? enclosure in middle of magnet pole.

Can anyone explain the logic of this????
Thanks in advance...

[dtm1962]

For clarification there is a self-adhesive square "patch" that is secured to the back of the magnet that seals the vented pole piece.

I would assume that uneven loading of the dome versus the cone of this midrange would be detrimental?

Any logical explanation of why Thiel did this.....

I have replaced the 13M8621 midrange (fs =112 Hz) with the updated 13M8640 (fs=64 Hz) and was wondering if it was done to load the driver since it does not have the excursion/power handling capability as compared with this Rubber surround driver.

The 13M8621 had a paper-like surround (better coupling in midrange versus losing excursion capability as a trade-off)

Any explanation would be helpful as I was going to remove the square patch that I installed/transferred from the original failed drivers.