However, this is just a rough guide. Valve has stated in the past that when they made maps, they watched where engineers wanted to build and just put ammo near them. So just try and get some spots in for a1 and fiddle from there. Offensive engineers usually have it tougher, but they can usually find somewhere to play without you needing to design for them.
That also has to do with the balance. Sentry spots generaly are in the same spot as that influences the balance alot better. A static place means you can often go for blind spam and have a certain chance of succes.
They also provide the part that a newbie can easily pick the same spot an expert would. It gives a low learning skill yet has a decent effect. The expert in this part is mainly better at ensuring survival/prolonging combat.
They are logical, when the players pick spots and arent decided by ammo it gives a good feeling for an engineer. You might have a strong spot at a location that doesnt feel comfortable, but that will just end up into frustration. And it also requires you to know the map, again the point that makes the expert getting a huge advantage.
You also want static nests for teleporter reasons. If you dont know where the tele will be you get the issue that you could confuse those that come out of it and make them walk backward. This makes it counter productive. With static positions you ensure that players dont get lost.
And last. Instead of 10 potential nests per point having only 3 ensures a more static flow. If an expert could pick a spot a newbie wouldnt get trying to balance between those 2 groups is somewhat impossible. The only thing that is acceptable is that an expert could choose between left, right, a little more to the front or back. As long as its close to the normal spot it will still feel natural to the players. If they can build at 10 spots you need to check all 10 of them, its effort you dont want to give those that are attacking. You often want them to have a clear view as they are already the one taking the risk of approaching.