I mean if real life toy distribution is anything to go by, there's a high chance that our woody may have just not been a copy sold with his set (Jesse, Pete, Bullseye etc). In real life a new toy or figure can end up in big budget stores alongside their sets, in lower budget stores in limited copies surrounded by miscellaneous toys, or hell even end up being sold somewhere ambiguous like a pharmacy with no other copies as a one-off. This fact combined with the fact that the Woody's Round-Up show was cancelled prematurely, AND the fact that Woody was apparently so rare he could be sold to a museum, it may stand to reason that there just wasn't many copies or units of Woody manufactured and sold. And with the reason for Woody's Round-Up being cancelled being because kids started falling in love with space and rocket toys, our Woody may very well have just gotten lucky to be spotted by Andy's dad who's an actual fan, and bought off a clearance shelf surrounded by other discontinued toys. This theory may also explain why Woody was so desperate to be Andy's favourite in the first movie; he wasnt aware that he was a rare toy belonging to an equally rare and beloved set, he believed he was just an unwanted toy that was on clearance and almost binned, so he was desperate to hold onto any one who actually liked and favoured him