Over 30 years working as an Estate Agent in the UK I witnessed this on many occasion and have heard many reasons as to why people do this.
Sometimes it's just because they don't want to upset the vendor by saying to their face that they didn't like the property/view/decor (FTB's are notorious for that one because they can't see beyond the decor and don't realise it is easy to change).
Other times however potential buyers do feel under pressure to make a decision on the spot so say things they think they are supposed to, when really they need time to think and compare the property with others they are or have viewed.
I used to tell sellers that although its good to get an offer, that is all it is at the time and to think along the lines of:
1) They might say they have a 'mortgage in principle' but that means nothing other than it is based on what they have told the broker but nothing has been verified.
2) Be pleased, but don't get too excited as due to the UK system where a purchaser or seller can withdraw anytime prior to exchange of contacts without financial penalty, it seems sometimes that the moving day will never come then you can get stressed but there really is no need.
3) Don't always take as gospel the agent who says "they want to be in within 6 weeks" as he will have also told the purchaser that you "want to be out within 6 weeks", the corporate companies in particular use this tactic to keep the momentum going as it is a fact that when things start to drag on sales the 'fall out rate' increases as people start having second thoughts, also it helps with the agency cash flow.
4) Avoid direct contact with your buyers, unless you are a proficient salesperson, negotiate everything through your agent and keep it in mind that this is a commercial transaction and not a hunt for new best friends!
When we purchased our last UK home it took over 8 months from offer to completion and although it was frustrating at times we took the view that it would not affect our lives unduly and if it wasn't meant to be then so be it!
At the other end of the spectrum, as an agent I showed a couple around a property that they fell in love with so much that contracts were exchanged less than 7 hours later so it can go very fast if the circumstances dictate and everything falls neatly into place.
Just remember there is a buyer for every property and that for every one that slips away, there will be a better purchaser just waiting for a chance to buy your property.
Most of all relax as you don't want to be so stressed out when you move that you don't enjoy the experience of relocating to Turkey.
Good luck