Hi John,
Now that we know the aircraft type it is a bit easier to give you some answers.
First of all you will find that VNAV, and specifically VNAV descent, is the least understood part of the automation in the 737. You'll come across people who say things like 'I do not trust VNAV', when they really mean 'I do not understand VNAV'. To find out how it works you have to play with it, but as a lot of captains don't really understand it (your profile suggests you're a first officer) you have to find the ones who do and will allow you to experiment.
How it behaves exactly in the situation you describe depends on a few things. One of them, over which you have no control, is how the FMC is set up. It can be set to full geometric path or approach only. I think most operators use approach only, but the other option does exist and complicates matters greatly. There are two ways to find out. You can look it up (but not change it) in the maintenance pages of the FMC or you can tell from the ND. When you're in the cruize the ND will show a T/D. Once you initiate your descent to waypoint A as you describe and provided this forces the aircraft into an early descent, have a look at the ND and see if it shows 'T/D FL180' on the ND somewhere after waypoint A. If it does, your FMC is set up for approach only geo path descent. Worse case scenario is that you fly a mixed fleet of pre-loved airframes and they are not all set the same depending on what the original operator specified.
If the FMC is set to full geometric path descent the behaviour of the aircraft will differ depending on whether you press ALT INT before or after reaching FL180
The ABOVE restrictions are taken into account when calculating the descent, but only influence things if they put you high on profile after the relevant waypoint. It might show a 'Steep descent after XXX' message in the scratch pad.
If at any time you want to know if the aircraft is in an idle descent or a geometric path descent, the easiest way to find out is to press the SPD INT button (provided you have that option). If the speed window displays a speed and the FMA shows 'VNAV PTH' you're in a geometric path descent.
It is difficult to answer your question without being there as it happened as we are dealing with too many variables and we also depend on your exact recollection of what you did in what order. You have to find the captains who understand VNAV and ask them to explain to you what the aircraft is doing as it is happening.
Although different Boeing models broadly follow the same philosophy there are subtle differences in the implementation. When asking these type of questions make sure you include the exact type so the answers you get are relevant to the type you fly. Rudestuff's profile shows the 757 as the type, and I am sure the information provided is correct for that type, but the 737 behaves in a slightly different manner.