You could ask the same question about anything e.g. Why do two different makes of cars of the same spec, have different prices.
In the example of monitors, the spec's might be the same, but the quality of materials used in manufacture will differ. Also, one make may have tilt/swivel facilities whereas the other might not. The only way to really find out the difference, is to compare them side by side when they are in operation. Also,it is better to buy from a manufacturer with a well known name and a good after-sales service.
They say that most faults occur in the first couple of months use.
You should be prepared for a few dead pixels on the less expensive TFTs. This has a couple in one corner. Unless they are right in the middle of your sight line or very prominent, they are a hazard of buying at the cheaper end of the market.
To sum up.
Go and have a look at what's on offer, if you like it and it's in your price range. Buy.
You will get it cheaper on-line, but some-one will have to be at home to accept delivery, about a week or so later.