Without reading the contracts, knowing where they were signed, etc., a pretty universal general principle is that if you make a promise to your client, you will be liable if you do not live up to that promise. Some of these promises will be expressly in the contract and some of them may be implied by law.
If you agree to deliver a reliable and accurate system (this is essentially a warranty), then if the system fails to meet that, then you do have a liability exposure there. What constitutes a "reliable and accurate system" is open to interpretation and will affect the extent of your liability, but the point is that the door is open.
Instead of disclaiming the giving of any warranties, often contracts will contain a limitation of liability clause, which caps liability. So, while a software developer may warrant that their system will do what they said it will do, if it doesn't, the most a customer can claim back from them is the capped amount. It is common for this to be limited to the amount the customer paid for the software.