This is meant as more of a GQ, but there might not be any hard factual answer, so I’ma put it here.
“If you donate now, your contribution will be matched by a challenge amount from the ACME Corporation - so your contribution will be doubled.”
These ‘matching challenges’ are doubtlessly effective - else why so many of them - but I can’t bring myself to believe that they’re totally honest. It seems more likely to me that ACME has donated $X anyhow, so the charity will only be missing your smaller contribution if you never send it in.
Totally honest or not, it won’t stop any of my contributions, but I’d frankly enjoy learning that one of my gray suspicions is wrong.