Hi Sanger, I'll have a go at explaining if it helps.
Consider the altimeter a barometer, that's all it is. It senses pressure wherever it is located. To get a readout from it, you need to tell it where the ground is. You do this by telling it the pressure on the ground, either at the airfield (QFE) or at sea level (QNH). It then works out the difference between the two and gives a readout of approx 30ft for every 1 mb.
You must not lie to the altimeter about where the ground is, or the readout it generates will be incorrect.
If on Day 2, the pressure on the ground at sea level has changed to 1010mb, but the subscale is still set to 1000mb. You are now lying to the altimeter. The altimeter still thinks sea level is at 1000mb. But it is actually 10mb further down in the atmosphere. 30ft x 10mb = 300ft therefore the sea level is 300ft further down than your altimeter thinks it is. It therefore under-reads by 300ft and shows 124ft.
On Day 3, the opposite has happened, the pressure has changed again and we are once again lying to the altimeter. We know that it is over reading this time by 300ft (724ft-424ft), therefore it must think the sea level is further away that it actually is. It is out by 300ft/30ft=10mb, and thinks sea level is at 1000mb, so the actual sea level pressure must be at 990mb, some 10mb further up in the atmosphere.
Hope this hasn't confused you further - you may find it useful to look at some diagrams on this if you can get hold of any, it may be easier to visualise.