following on from dragons links, i was just thinking about some possible theories…
(i dont know if they are true but the theory seems a bit plausible) 
- with blank cds, some manufacturers included more track length which was usable in creating a cd that could hold more data. (for example there were the usual 74min blanks, and then some 80min blanks, but some 80 min blanks actually let you store about 81mins worth of data. then some were 99min cds but each came with certain options or restrictions such as which player or reader would recognise them fully)…
… with this in mind… is it possible that some makes of hard drives are simply called xTB drives because they have at least xTB of space, but that maybe the have a little bit more? (if so, is that extra space essentially balanced or cancelled out by the SDR/DDR process?)
- also, is it possible that the original drive had some bad sectors that were unusable (but that the drive lasted as long as it did without any more bad sectors appearing) but that this slightly reduced capacity meant that the final total usable space for data was still below the 16TB mark (and it was also balanced out via sdr/ddr?).
theoretical ramblings aside though, here are a few quick thoughts on your recent numbered questions loius: (and where possible, the thoughts have about 1 word by contrast…at the start) 
- Possibly, but you would still need to manually keep track of how much data you have on the whole drobo so that you do not risk overfilling.
(i say possibly because while each volume is actually being served via the same hardware unit, volumes or drives on most devices generally tend to slow down as they become full, especially near the end, but you probably wont reach, and probably wont want to reach that level of “fullness” with your current setup).
- probably
i think it depends on how you do your backups, and what tools you use.
for example, regardless of the number of volumes i have, (and seeing as i actually require, and prefer, a maximum volume size of 2TB, i have a fair few across my drobos), i still manually select which file/folders to backup, from which volume at a time, and which destination folder to use. (depending on what is being backed up, i also use a very good windows tool which is part of the syncback range, which allows me to set-up backup profiles, but rather than simply backing up each virtual volume at the volume level, (unlike when you need a full single hard drive image via acronis or similar), the profiles are set to include all files or folders within the defined root, but with specific exclusions on certain special folders, (or some folders that i am not wanting to include in that particular backup profile).
- Dashboard is your friend
as far as i know, windows will report the correct amount of Used space, even if the max volume size with free space is higher. for example you could select all files, and do a properties to get the used space (but ideally would use the largest reported sizing in properties dialogue, and would be able to view all potentially hidden files or system folders etc)
The main volume (drive letter) window could give properties too, but i think the best way is to see how much free space is left, via drobo dashboard.
- embrace - i would try to embrace this particular change (partly because it is not actually that bad and can have some benefits as i might have mentioned on some other related posts, but also mainly because you are so close to the 2nd volume borderline anyway, that you will most likely need to have another volume if you significantly expand any more)
another user was able to use a windows server and another 3rd party tool (which i dont think is free) to essentially merge multiple volumes across multiple drobo units, into 1 large pool - but i honestly dont think that will benefit you at leasst not at this stage, and can also add a lot of other variables in the mix.
- relax (i think as long as you follow step1 and use dashboard for your remaining total space monitoring, then DDR should be able to still cover you in terms of 2 drives)
(i was going to say “frankie goes to hollywood” as the 1st word