I think the problem is that some TOCs apply their own criteria - which they're free to do, so as long as they exceed the legal minima and can be justified on health and safety grounds (to avoid discrimination claims). For reference, here is the new standard:
http://www.legislation.gov.uk/uksi/2010/724/contents/made
Go straight to Schedule 1 at the bottom...
For the record, the unaided visual acuity requirement
has been removed, so it's my contention that for many the new requirements are less strict. The unaided acuity requirement was only ever there (so I've been told anyway) because of concerns about high prescription strength lenses causing distortion in the peripheral field of view. As such, this is a much more sensible way of doing things, they're saying that only those with lenses that are below a certain level of correction can be passed (which happens to be +8 for short sighted). The old unaided acuity standard was a bit of a bodge attempting to find the same thing.
Confusingly, the RSSB are updating their legislation at the moment and intend to split the drivers requirements away and put the requirements for guards and despatchers etc. in a new publication. That new publication will use the old standard for vision. So, in theory someone could be a legal driver but not meet the requirements to be guard. If I was to apply, it would apply to me, I can meet the TDLCR regs but can't necessarily meet the old vision standard. Hopefully they will correct this post-consultation, but who knows.
And as I said, some TOCs are arbitrarily putting more stringent requirements on top of the legal minimum, which only serves to muddy the water (and it looked pretty brown beforehand...)