Honest Question Re Traveling Triplets

Third-party reference:
http://mentock.home.mindspring.com/twins.htm
"To avoid accelerations in the thought experiments above,
we can simply make the second Bob frame into a 'messenger'
Carl that never accelerates, but passes by Bob as they set
their watches together. Messenger Carl then travels to Ann
and compares watches as they pass each other. That makes it
clear that there are three ... inertial frames involved."

* Yes, there are _no_ accelerations (using triplets).
* Yes, SR's math yields the correct numerical values.

However, physics calls for a physical explanation, and
yet math cannot provide such (as everyone knows).

The only explanation given at the above web site is the
fact that the triplets are in different frames, but this
does not tell us why being in different frames causes
triplets to age differently.

So here is the question:
What is the "official" physical explanation for the
observed difference in ages for traveling triplets?

Disclaimers:
This is not a trick question, nor is it a leading one;
it is presented purely in the spirit of sincere enquiry,
sans any speculation of any sort.

.

Relevant Pages

  • Re: The real twin paradox.
    ... SR does not describe time compression. ... A's change it frames will ... Only speed affects clock rates, ... The physics.nmt.edu page describes accelerations in SR. ...
    (sci.physics.relativity)
  • Re: I dont get it
    ... yourself is Why does merely being in different frames cause one ... Please, Daryl, we have already eliminated all accelerations. ... odometers are NOT like clocks; ... a clock moving through space does not rub on anything ...
    (sci.physics.relativity)
  • Re: The real twin paradox.
    ... A's change it frames will ... The physics.nmt.edu page describes accelerations in SR. ... paradox described in the OP for SR. ... the twin observes the time dilation of the ...
    (sci.physics.relativity)
  • Re: I dont get it
    ... can simply make the second Bob frame into a "messenger" Carl ... compares watches as they pass each other. ... that there are three distinct inertial frames involved." ... The theory's description of inertia is on ...
    (sci.physics.relativity)
  • Re: The real twin paradox.
    ... SR does not describe time compression. ... A's change it frames will ... How does special relativity deal with accelerations? ... Only speed affects clock rates, ...
    (sci.physics.relativity)