If/when a barrel was shortened, sometimes the front pipe was moved back to accommodate the change, but not always. If the rifle was built with pipes evenly spaced, then rearward movement of the front pipe will of course _shorten_ the space between the mid and front pipes.
With the question the OP asked, that wasn't the issue. The original question was about the space between the middle and front pipes being _longer_ than the space between the entry pipe and middle pipe.
I have an original Early Deep River NC rifle that has 13 1/2" between entry and middle pipe, and 15" between middle and front pipe, measured from the center of the pipes. 46" barrel. I assume it has been trimmed at the muzzle, but the pipes were not moved. Muzzle cap was just moved back a little. Pin now goes through the muzzle cap.
Also have an original East Tennessee SMR that has 12" between entry and middle pipe, and 15" between middle and front pipe, measured from center of the pipes. 43 1/4" barrel. Trimmed at the breach, but pipes were not moved. Cap was just moved back a little. Two sets of pin holes, from the barrel having been moved.