No, it was the
height of the characters. Using a font with no descenders (hanging bits like on "g" and "y" is a long-established trick for getting more height of dot-matrix displays - otherwise you have at least one row of pixels that are only ever used by a handful of letters.
Specifically, per the
Rail Vehicle Accessibility (South West Trains Class 458 Vehicles) Exemption Order 1999 (No. 2404):
(Note that
5(6)(a) was later amended to be 31 July 2006)
Regulation 13(7) being:
but the original displays had a 32 mm character height.
Ultimately new displays were fitted to the fleet - you can tell if you look closely at the curved centre-line panel in the ceiling, which curves ever so slightly over the larger panel. ScotRail's 334s were not modified - for reasons that I do not know - and they still have the original displays for comparison.
I should mention specifically for
@TT-ONR-NRN's benefit that the no-descender fonts are probably much more common than he thinks; given that they're used in two different applications on Pendolinos and that he'll also encounter them on both Alstom and Annax systems should he visit Melbourne again.
Ah, you got there ahead of me! But yes, there were five other regulations with which non-compliance was only temporarily exempted. The explanatory note with the 1999 order lists the full set as: