What's in a word?

Question for you... my site now has 170 pages (although I'm adding constantly). They're tiered in the following way:-

Homepage
Airport Transfers by Area
Town pages (for example Christchurch Airport Transfers with pricing tables of the various airports)
Then each pricing table links to a final page which is a page regarding the individual airport transfers (for example Christchurch to Heathrow)

This works well because it gives Google a HUGE amount of keywords to find on different pages and we're ranking really well for most places... generally between page 1 and 2 depending on the town.

My question is about the individual airport transfer pages which all follow the same format. I noticed earlier that for quite a few of them were page two for searching the page name, until I found out something that I can't explain and I'm hoping someone can!

Each page has a meta description, title, url and keywords relevant to that page, here's an example:-

Christchurch to Heathrow:-

URL:- http://www.castle-cabs.co.uk/taxi-fr...h-to-heathrow/

Title:- Taxi from Christchurch to Heathrow

Keywords:- Christchurch, Taxi, Cheap, Heathrow, Airport, Transfers, From, To, Taxis

Meta Description:- Cheap Taxis from Christchurch to Heathrow Airport, Christchurch Taxi Service between Christchurch and Heathrow, Cheap Christchurch to Heathrow Airport Taxi Transfers

Now... if I type in "taxi christchurch to heathrow"... we're ranking page 2, number 8. It doesn't show the meta description, it shows a page snippet taken from about halfway down the page... BUT...

If I type in "Cheap taxi christchurch to heathrow", we're page 1 number 6. Obviously this will depend on where you're searching from, but this is with a cleared cache and history on a location set to "Bournemouth". What I find interesting though is that by adding the word "cheap", it now brings up the meta description and highlights 12 words...

So is there a reason for this? Is it perhaps because "Cheap" starts the meta description so I should alter these to start with Christchurch instead?

Grateful for your thoughts