Why do grape-flavored foods taste different than actual grapes?
Concord grapes, which most grape jellies/jams/preserves in the US are made from, are derived from the (US-native) "fox grape" (Vitis labrusca) rather than (Europe-native) wine grape (Vitis vinifera). Common table grapes (the ones eaten as fresh fruit) such as Thompson seedless are also derived from Vitis vinifera wine grapes. 

Fox grapes have a "foxy" taste character, which is a result of the presence of the naturally occurring compound methyl anthranilate. Methyl anthranilate is a rather simple compound, and is used in many situations as an "artificial grape flavor". In many cases "grape flavored" candies, drinks and medicines are flavored not with grape extracts, but with synthetically produced methyl anthranilate. As such, these artificially flavored foods taste like Concord grapes (fox grapes), rather than table or wine grapes.

While it would be possible to come up with "artificial wine grape" flavor, the flavor profile of wine and table grapes is not dominated by a single compound, as fox grapes are. Therefore, any such "artificial wine grape" flavor would be much more expensive than artificial Concord grape flavor (i.e. just methyl anthranilate). As such, when companies reach for "grape flavor", they tend to go for the more inexpensive Concord grape flavor.

This also adds to the persistent expectation (at least in the US) as to what "grape flavored" means. Even if you came out with a wine-grape-flavored Jolly Rancher, many in the US would think it wouldn't taste right, as they expect grape flavored things to taste like Concord grapes.