Federal sentencing guidelines almost never ask for "fully stacked" sentences. The term you're looking for is "consecutively." Most of the time, all sentences are served "concurrently," or at the same time. On rare occasions, as you write, a judge will specify that the sentences be served consecutively, to keep an exceptionally bad felon behind bars for as long as possible. Of course, the prosecutor can always threaten to ask for consecutive sentences to bulldoze the defendant into accepting a plea.
I've never considered this before, but isn't that a bit of "the law of unintended consequences"? For instance, if I know that I can commit a single crime now and pay the full rate or commit multiple crimes and pay them off in parallel, why deterrent is there for me to limit my activities?
I've always hated a law that I've heard of (this is strictly hearsay, I've never actually verified it) where in some parts of the US, if you involve a gun in a robbery there's an automatic minimum sentence which is in line with the sentence for having a gun and committing assault; just by having the gun you're already in as much trouble as if you used it, so there really isn't any incentive to not use it if you think doing so could allow you to avoid capture and prosecution. If you're going to be charged for attempted homicide, it's not like taking a hostage is going to add to your sentence at the end of the day and it has the most narrow chance of getting you off the hook.
My SO is a paralegal and I'm always amazed at the perverse incentives that are built into the legal system since you always seem to be in for a pound when you're in for a penny. No doubt legislation passed by someone "tough on crime", but the public seems to support it. I've always thought that putting someone in a place where they stand to lose nothing is a rather poor position to put them in.
For instance, I had lost my license due to unpaid tickets (I thought I had paid them all off, but I must have missed a payment) and I was pulled over for a taillight being out. At this point I now had a suspended license for driving on a suspended license (aka, strike two before they pull it for 5 years in the state I was then living). I went to a concert with some friends and they all hit the booze a bit hard and when it came time to drive home and they were all too drunk, we let the least drunk guy drive. It was more dangerous than the sober guy driving, obviously, but we reasoned that if we got pulled over the law would take it easier on a guy with a first DUI, a wife and a kid than a guy with a third violation for driving on a suspended license. The math was easy; I got "caught" (I didn't know I was doing anything wrong) a second time and they took my license for a year for the second infraction; your first DUI infraction is an automatic 1 month suspension or so. We knew which crime was "worse" in this case by what would have happened to myself versus what would have happened to the drunk guy that drove us home. We also knew which was safer, but it's not like the cops would have taken that into account, so it wasn't really a consideration in our assessment of the situation.