A compressor works on pressure ratios. These ratios are calculated using something called the rule of squares which in its simplest form is used to design two stage compressors. Based on simple equations using this principle, the design engineer, in effect, couples two, 2 stage compressors to make a four stage. The relationship between the sizes of the stages produces the ratios. As with all stages, the third stage produces its specified pressure only after its contents are dumped into the fourth stage. This happens while the smaller fourth stage is bottoming. So, the third stage develops its rated pressure due to the compression of its gas load into the smaller volume of stage four. The fourth stage produces pressure in accordance with the load. In other words, the pressure inside the fourth stage rises in concert with the back pressure or that inside the SCUBA tank. The spring loaded discharge valve of the 4th stage plus some level of back pressure will ensure that some pressure will develop inside that cylinder at start and when the back pressure rises a little bit and the piston stops banging it is operating normally. However, it will not hold all of the pressure which should develop based on ratios until the back pressure rises to a certain level. This is the reason that stage pressure varies. When the third stage dumps its load into stage four some of this gas blows on through while the piston is falling and simultaneously the balance of internal pressure serves to drive down the piston. When the pressure at the back pressure valve or inside the scuba tank approaches the third stage design pressure (1100 psi) there is higher back pressure on the discharge valve and no more blow through at the fourth stage discharge valve; so, the fourth stage pressure rises while the third stage should remain relatively steady. At this point, the specified interstage pressures should all be close to that listed in the manual. However, due to valve leakage or overlap among the stages these values could continue to vary somewhat, rising with delivery pressure. The point of this is that the fourth stage load is shared, so to speak, with the third stage until delivery pressure rises to a crucial level. I think that this is what you are referring to.