I come up with splainins all the time as to how different characters played by the same actor within the same show can look so identical. It's usually due to plastic surgery, shapeshifters, or a case of identical cousins. My personal favorite is that of identical half-brothers whose father played around; that sort of thing.
Unless it's otherwise stated within shows, however, I think that for the most part these characters who resemble each other must have something about them that differentiates them in the eyes of other TV characters.
Otherwise, wouldn't have Lt. Columbo noticed that there were three murderers who looked like the late Jack Cassidy; three killers (and the father of another) who all resembled Robert Culp; not to mention all those fussy little men running around Los Angeles who could be copies of Vito Scotti?
At least all four murderers played by Patrick McGoohan had something different about their looks so that they didn't resemble one another.
Character actors like Morgan Shepherd and Jack Elam revisited the Ponderosa several times over the years. They couldn't all have been the same guy given a new identity by Fabian Lavendor.
And Cabot Cove, Maine, is too small a town for some people to keep showing up with the same face without JB Fletcher noticing.
Perhaps it's some kind of aura, more likely something physical about their appearance, that sets them apart from those characters played by that same actor in the past. We just can't see it.
This would splain why Don and Charlie Eppes could watch 'Taxi' on TV and not marvel over the amazing resemblance between the cabbie Alex Rieger and their own father... even if a quarter of a century now separated the two in looks. Alex would still look like Alan Eppes in old family photos.
As for why 'Taxi' exists in Toobworld as a TV show as far as 'Numb3rs' is concerned, here's my splainin:
The Brothers Eppes were watching one of the very first reality shows about the inner workings of a business in New York City, in much the same way that there's a reality series about a southwestern airline running today. That version of 'Taxi' (perhaps inspired by the Harry Chapin song) focused their cameras on the Sunshine Cab Company to document what life was like for these cabbies.
So what we saw as a fictional comedy series here in the Trueniverse played out on TV screens in Toobworld as factual documentary programming.
(I didn't see that 'Numb3rs' episode. Did we hear a laugh-track? If so, easy splainin: post-production technical error.)
[Thanks to TVGal Amy and her readers Carl and Patty for the tip.]
'The Wild, Wild West
'Murder, She Wrote'