Sorry to tell you this but I have gotten 2 and sometimes 3 Saltpeter drops from the same spot. And just because you don't get Saltpeter on the first swing does not mean there is no Saltpeter there.
PLEASE NEVER ASSUME THAT BECAUSE YOU DID NOT GET SALTPETER ON THE FIRST TRY THAT THERE IS NO SALTPETER THERE. IT IS ALL UP TO THE RNG.
There are a couple different things going on here:
1> When dungeon mining, if you are at GM Mining, if there is a niter deposit to be found within an 8x8 chunk, it will be discovered on the first strike. If you're at less than GM mining, the results of the first strike are not certain.
2> For dungeon mining, once the chunk has been checked for saltpeter, success or fail, it won't be checked again for 5 minutes. So, if you find a deposit and it takes you 5 minutes to finish mining it, then you can immediately re-check that same chunk and could very well find another deposit.
3> For boat mining, the chance of finding a deposit is checked for each swing of the pickaxe, but the odds are a lot lower, and the resulting niter deposits will be a lot smaller.
Basically, boat mining is much safer and easier than dungeon mining, so the saltpeter reward should be less. But, "prospecting" a lot of chunks for saltpeter would be much slower on a boat. So, when boat mining, the intent is that you occasionally get a small niter rock when mining normally, kind of like mining up a gem. With dungeon mining, the intent is that you can "prospect" for saltpeter specifically to get it faster, or just get some as a bonus when mining normally.