Perhaps I did miss your point; I work for an airline and we do get people who think that just because they've gone through a checkpoint once and then back out that they shouldn't be screened again, or that just because their ID was checked at the ticket counter it doesn't need to be checked at security (as if they couldn't give their boarding pass to someone else in between). In this case, you have to realize that in the real world no two pieces of equipment are exactly identical. You probably had an amount of metal on you that was right on the alarm/no alarm border; if you walk through at slightly different speeds, or if one machine is just a hair more sensitive than another, you can get different results. You may also have enountered different brands or models of magnetometer. It is even possible to walk through the same magnetometer multiple times with the same pieces of metal and get different results each time if the amount of metal is marginal; it's just part of operating in the real world.
Here's where I'm at on the whole airline security thing: I'm far less worried about the competence of the TSA agents than I am about a few other things. True incompetence is fairly uncommon at this point and is not predictable enough for terrorists to plan for and take advantage of. There are some systemic issues and holes that worry me a great deal; most of these have not been widely publicized and I am not going to begin that here.
One final note: it does not matter that you are a grandmother in your 50's. If exceptions to screening are made in any way on any kind of a consistent basis, all the other side has to do is start recruiting people who fall under the exemption to smuggle items through security for them. There are plenty of women in their 50's (or members of any other demographic, for that matter) willing to do such things, if given the opportunity, for a variety of reasons.