It's because they haven't put the weight in. If they put weights in the listings the prices are more realistic. Often the weights aren't put in because they don't know they are listing outside the UK/US so don't see the need.
Even though most of my stuff is under 500g I still put the weight range in. I have seen it be automatically entered at over 3kg for something that weighed at 15g when I've created a listing from scratch. If my postage was set up to be calculated rather than flat rate, buyers would be seeing an awfully high cost! Given how glitchy eBay is at times, I don't want to see postage get changed from flat rate to calculated due to a glitch and then wonder why sales suddenly die off.
