I'll say it again: the outcrops are the only things made up of the planetary composition - the meteorites are dropped onto the surface from space.
Now this thread was supposed to be about whether the published percentages squared-up with the reality. They don't.
Here is an example from this evening, I got fed up not getting any DMW scans so went for a wander on the little moon nearby. By smashing up a sequence of outcrops I picked up in the space of half an hour 2 chunks of antimony and 6 chunks of arsenic. The given percentages were 1.2 and 2.4 respectively (I ignored the mass of tin and niobium). This is a tiny rocky moon:
It seems to me that Arsenic is just one of those weird things in the game that you either trip over all the time or just get frustrated hunting for it. My conclusion is that the percentages are just not at all reliable.
What was the percentage metal of that little moon-let?? Your picture doesn't show that information.
Go here <<<< https://drive.google.com/open?id=1cJLQuoX3T481wmGKU1Ue4qUYkJkxIj02 >> and add up the percentages of materials listed on the two 100% metal planets.
They will add up to 100% +/- 1%
They means that the percentage show for Arsenic MUST be the same percentage of the planets total composition.
Now pick ANY non-100% metal planet and unlike the two in the folder I linked, there will be some % composition of rock, ice, or whatever and then a separate % for the metal. You add those two together and you will get 100% +/- 1%
So, the information I have been trying to convey applies directly to your question about the reliability of the percentages listed in the materials description. You cannot just look at the percentage of the material, You must also look at the percentage of the TOTAL composition that is metal and then work to the ACTUAL percentage of the specific material that is on that specific body.
So, what was the percentage metal of the little moon-let?? At 2.4% arsenic on a 10% metal body you would likely struggle to find any. While on a 33% body what you have described is entirely possible. On a 100% metal body is is almost guaranteed.