Exactly. You can't combine authorizations. (Well, almost--with Visa, some POS systems--again, not all!--allow you to do what's called an incremental authorization, which allows the merchant to increase the amount of the original authorization, but it can't always be done and of course only works with Visa.) So when your charges are settled at the end of your stay or rental, the system picks one authorization and applies the charges against that. (And if your charges are more than any one authorization and the merchant can't do an incremental authorization to make up the difference, the merchant can either manually do several smaller charges and use up your existing authorizations or--what is more likely--just run a new authorization for the lump sum and ignore the old ones.)

Even among the broadband-based credit card modules built in to some POS software, there's room for agent error or confusion. (For example, the one I work with requires the agent to manually select the previous authorization, which often results in none getting selected.) A well-designed software suite (which ours is not!) will work well, but unfortunately, most niche market software companies simply don't have the resources to design a good UI and do good human factors design (even the majors like Apple and Microsoft get it wrong all the time, and they have thousands of people working on getting that stuff done right!).

Oh, one other complicating factor: different card-issuing banks hold authorizations for differing lengths of time. Some (as evidenced in this thread) stick for 3-4 days and others won't go away for a full month. Debit cards are usually on the shorter end (which is one of many reasons why many hotels and rental car companies refuse to use them or require additional qualification steps). So for some people, reversing an authorization at the end of a stay is redundant, because after a five-night stay, any unclaimed authorizations may have disappeared anyway. (I believe the merchant agreements with the various acquirers and other institutions involved in credit card transactions [it's WAY more complicated than you'd think!] allow a merchant to submit payment against an authorization up to 30 days after the authorization was requested, even if the authorization has appeared to fall off and the charge would cause the cardholder to go over his or her limit.)

Really, the whole credit card system needs an overhaul from the ground up. The PIN-based ("online") networks are a good start, since both authorizations and fund transfers happen in real time and don't go through as many steps, but they aren't feasible for hotels, rental agencies, online merchants, and other merchants who need the ability to run card-not-present (and cardholder-not-present) transactions or need to perform incremental charges for things like extensions, mini-bar charges, and vehicle damages--but that's a whole 'nother ball o' wax!