Bought a threadless bottom bracket - doesn't fit in the shel

I have an old (2004 GT) MTB with damaged threads in the BB shell that I thought I'd try and resurrect. I bought a YST threadless bottom bracket here:
http://www.ebay.co.uk/itm/Sealed-Bearings-Threadless-Bottom-Bracket-For-Damaged-Frames-Threads-110-127-/261543679095?var=560418703178
The non-drive side cup bit slid in fine but the drive side just wouldn't fit. I tried forcing it but it was impossible and even with a hammer and plenty of grease no progress was made. It ended up damaged and was subsequently binned. It was maybe 1mm or so too big in diameter.
I'm confused, I know bottom bracket shells have different widths but I thought the diameter was consistent so I don't understand why it wouldn't fit? The bike was supplied with a run of the mill Truativ chainset and a generic cartridge BB so this looked like a straight replacement.
http://www.ebay.co.uk/itm/Sealed-Bearings-Threadless-Bottom-Bracket-For-Damaged-Frames-Threads-110-127-/261543679095?var=560418703178
The non-drive side cup bit slid in fine but the drive side just wouldn't fit. I tried forcing it but it was impossible and even with a hammer and plenty of grease no progress was made. It ended up damaged and was subsequently binned. It was maybe 1mm or so too big in diameter.
I'm confused, I know bottom bracket shells have different widths but I thought the diameter was consistent so I don't understand why it wouldn't fit? The bike was supplied with a run of the mill Truativ chainset and a generic cartridge BB so this looked like a straight replacement.
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What happened when you tried the NDS part in the farme drive side and vice versa?
retired 9.6kg Carrera Kraken
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Definitely nothing in there. The non-drive side cup went in just fine on either side.
Did the DS cup go into the NDS of the frame?
retired 9.6kg Carrera Kraken
The Carrera Hardtail combined thread - come on all you Carrera's!
The Sons Scott Genius RC20 build