Maybe it doesn't comply with the recommended interrogation sequence? There's a flowchart in the SD specs showing you how to properly differentiate. It's backward compatible. If you (as a designer) just initialize and go, instead of probing and verifying behavior, you may end up using the wrong command set. Just a guess.
Uh... yeah, OK. Point taken. I guess I was looking at it the other way around. An SDHC-compatible reader must... as in, per the spec, absolutely must... know how to tell the difference between SDHC, SD, and MMC cards. But you're right, there's no guarantee that a new card would work in an old reader that's not aware of fixed block addressing and whatnot.