According to the BBC website it's all about the difference between less and fewer.
The new wording was suggested to Tesco by language watchdog The Plain English Campaign. Tesco said the change would be phased in across its stores."Saying up to 10 items is easy to understand and avoids any debate," said a spokesman for The Plain English Campaign.But is this change important? Does this distinction actually matter? Everyone knows what's meant by "10 items or less". In fact, isn't "Up to 10 items" actually less clear? Does it mean up to and including 10 items or up to the point where 9 becomes 10, but no further? In fact, does it matter at all when most people actually pretend they can't count and just take 20 items?
"Fewer" should be used when you are talking about items that can be counted individually, for example, "fewer than 10 apples"."Less" is correct when quantities cannot be individually counted in that case, e.g. "I would like less water".
The debate - oh yes, there's a debate about this - is followed up in comments on the Daily Telegraph's website, where you can see some very poor spelling and grammar from those who claim to be grammar experts. Tut tut...
ENA5 - Language Change
ENGA3 - Language Explorations