Why is prospecting "a chance," mechanically speaking?

[Venusgate]

I recently dropped into a cave over a 11% copper chance to find 5 whole blocks of rich copper. Is it that caves generate after ore and prospecting data, so they can "wipe out" nodes?

[MKMoose]

Prospecting data (specifically density search) is not concerned with what ores actually end up generating. The game generates a random distribution for each ore type that can be detected by the prospecting pick's density search, and this distribution is used both when randomly generating the ore and when the prospecting pick gives you a reading. That is, the density readings inform you about the actual chance that the game uses to generate the deposits, but whether that ore has actually generated under the reading is a separate matter.

 

1 hour ago, Venusgate said:

Is it that caves generate after ore and prospecting data, so they can "wipe out" nodes?

Caves generate first, then ore deposit generation replaces rock with ore blocks. Air is treated as a different block, so it doesn't get replaced.

Either way, the end result is as you're assuming - if a cave happens to pass through a deposit, it effectively removes a bunch of the ore blocks. I've recently found a cassiterite deposit with exactly one ore block in a cave, which was quite amusing. Your case seems pretty unlucky, since deep copper deposits (I'm assuming you're talking about a deep and not surface deposit) tend to be much larger than cassiterite deposits: ~14 blocks average radius, and can generate up to 3 blocks thick. It is nonetheless entirely possible for a cave to remove most of it, provided the cave is large enough.

[Venusgate]

2 hours ago, MKMoose said:

Your case seems pretty unlucky, since deep copper deposits (I'm assuming you're talking about a deep and not surface deposit)

80 deep. it's the second cave tendril I had to breach before i found the copper.

[MKMoose]

2 hours ago, MKMoose said:

~14 blocks average radius

I just noticed, small correction on that part: it's 7 blocks average radius, while 14 blocks is the diameter.