The numbers are lifetime, & the F-35's expected to be in service for a very long time. Yes, attrition is very low nowadays, not like the massive loss & death rates of the 1950s, but it's not zero, & when an aircraft type is in service for a long time, there'll be a few losses, even airframes wearing out. The intention isn't to buy 'em all in one go, or even to have 138 in service simultaneously. If we're not going to buy the planned number, surely the chance of buying some A as well as B goes down. Would we want just a handful of A?