Can't see anything wrong with your reasoning.
Have you tried moving the green knob round a few divisions each way to see if they are all out by two positions? Maybe someone repaired the scope and put the knob back on wrong!
I like your theory! but three clicks is always x10 on that knob, so the test would be if it hits the stops in the wrong place.
You might be onto something, this is where the knob stops
Although it doesn't keep turning as you move clockwise, it stops at the smallest value.
Is any input out by x10 on any timebase setting? If so perhaps the x10 magnification is stuck on? The button in the upper right of the picture doesn't appear to be depressed but there may be an issue behind the panel?
The button at the top you are referring to is only an x1/x10 for the external trigger input. On page 3-4 is a section that reads, Delay Multiplier: Multiplies the TIME/DIV as selected by A time range switch over the range x0.5 to x10 providing a delay range from 200nSec to 20Sec.
But I can't find any such control!
Take a look at page 2-1 of the instruction manual and verify that you do not have the time base vernier engaged. It allows you to vary the actual sweep speed from the timebase knob setting.
The horizontal magnifier is a 5x, not the normal 10x, by the way.
Whoever designed the front panel of that scope should be doing something they are more suited for, such as taking orders at a McDonald's hamburger shop. It's poorly labeled and grouped, let alone color coded.
And a properly designed oscilloscope would have an indicator LED that illuminated when a time base vernier is engaged - it would be labeled "UNCAL" or the like. (Same with vertical gain vernier.)
Yes I was caught by the vernier control at first too. When you rotate the A timebase control, the vernier moves with it, must be dragging. Also, don't forget this is a 30 y/o scope!