As you suspected: claw machines are rigged

Claw machines can be programmed to automatically reduce their grip strength to maximize profits, while allowing an infrequent full-strength grip to entice suckers.

From the instruction manual for a claw machine:

Managing profit is made easy, simply input the coin value, the average value of
the merchandise, and the profit level. The machine will automatically calculate
when to send full strength to the claw, games sent full strength will be
randomly selected from a group making it difficult for players to "predict."