Why does a 28 1/3 tyre fit, when a 27 1/4 tyre is too big? I've just bought these rims off eBay:

http://www.ebay.co.uk/itm/Mercian-built-Wolber-rims-Shimano-hubs-vintage-27-inch-good-condition-/270921291817?pt=UK_sportsleisure_cycling_bikeparts_SR&amp;hash=item3f142ad829#ht_500wt_1401

I've fitted a 27 1/4 tyre to the front, no problem.

But the rear only accepts a 28in Tyre, and the 27 is too big...

Why is this? And what size would this make the rim?

Many thanks

From what I've gathered here: http://www.ctc.org.uk/desktopdefault.aspx?tabid=3802

It would appear that some 28 tyres fit onto rims with smaller diameters.

Now, that raises a new question, why would Mercian sell a pair of wheels with different size rims? - Or wouldn't they?
European ISO 622mm (AKA 700c) tires are often labeled as 28" tires.  Confusingly, the very rare 635mm tire is also labeled 28".  This is unfortunate and leads to much confusion.  The common 27" tires are ISO 630mm, which means that you will almost always find that tires labeled as 28" are actually smaller in diameter than those labeled as 27".  

The sizing system for bicycle tires is a confusing mixture of "standards" from different companies and nations.  The definitive Sheldon Brown tire sizing article has more detail than you'll likely ever need.

If your 27" (630mm) tire is slightly too big for your rim, it's likely that the rim is 622mm.  It's probably worthwhile to visit your LBS; there are lots of standards and when it comes to tires you definitely want to make sure they are properly fitted.  Having a tire come off the rim while riding is an unpleasant experience.

As commented above, the difference in sizes between the wheels is probably because the wheelset didn't originate as a pair and came from different sources.