Actually, the non-holo thin stamp Base Set cards are usually not as thin as actual Thin 1st edition stamps we see on holofoil Base Set cards, other WotC era sets, or other languages. The width of the “1” is kinda in between an actual Thin and Thick stamp. Here an example of all three stamps (in the order non-holo thin Base Set; actual Thin stamp; Thick stamp from left to right - the non-holo thin Base Set stamp is graded, otherwise I would have put them in increasing order…):
So they didn’t accidently used the Thin stamps for some non-holo Common/Uncommon sheets, but instead it’s just a printing error like grey stamps, back imprinted stamps, etc.
Some of the non-holo thin stamps I’ve seen were also partially smudged with black ink, further explaining why it got a thin stamp. Like this Red Cheeks Pikachu at the top for example (not mine 
):
Thanks for the tag and link to my article, @waynesworld . And nice non-holo thin stamp Rattata, @ccrntrade ! 
Greetz,
Quuador