Why are options that cannot be traded in cents still show a last trade as non-five-cent increments?

Rules on quote width are complicated and there are a number of exceptions. Each exchange has its own rules.

In general, options in high volume securities such as SPY, QQQ, and IWM, are quoted in penny increments regardless of the price. For others that trade below $3, the increment is $0.01 or $0.05 and for contracts above $3, the increment is $0.05 or $0.10

A market maker is not rigidly limited to five to 10 cent increments. The legs of a combo order such as a spread can be filled at any price even though the legs are quoted in 5 cent increments.

Suppose I place an order to buy 5 verticals for 75 cents. I could get all 5 spreads whose legs are at the same price (-1.60 + 0.85). Or all 5 could be filled at (-1.63 + 0.88). Or some or all could be filled at different prices with some crazy fills like (2.88 - $2.03). So even though the quotes were at 5 cents increments, fills could be at one cent increments.