Car rental rates are up — finally! (I’ve made this jubilant proclamation before, only to see rates slide into a prolonged slump. So I won’t get too far in front of my skis.) Nonetheless, our June rates survey of the top 50 U.S. airports shows an average $6.37 gain from June last year, after six straight months of negative comparisons. Let’s be happy about that.

This is in light of fleet sales to rental being down 14.8% through the first half of 2017 — the biggest six-month sales drop since the Recession. Why are fleet sales down when travel demand is strong?

Based on hotel room demand and air passenger enplanements, the U.S. Travel Association predicts positive travel growth through October. As a point of comparison, at this point last year fleet sales to rental were up 8.3%. At the same time, the USTA predicted similarly positive growth, when its six-month Leading Travel Index (LTI) stood at 50.7. (This year’s index is even slightly better at 50.9.)

This only furthers the notion that car rental rates aren’t always directly correlated to travel demand. Instead, rate fluctuation is more closely tied to fleet size, and there are many factors coming into play.

First, auto manufacturers have been making good lately on selling fewer units to rental fleets, ostensibly because too high of a percentage of rental sales hurts residual values. This is ironic, because consumer incentive spending is at record highs, and there’s more of a direct correlation between incentives and used car prices.

Second, Hertz has completed its fleet refresh, which removed a lot of noise in year-over-year comparisons. In a recent investor presentation, Hertz also made a point that the company is shifting its fleeting philosophy to a more cautious approach. This promise appears to be materializing.

Third, as Avis and Hertz move a few percentage points away from repurchase programs to risk vehicles, overall fleet sales would dip as a function of longer risk vehicle hold times.

Fourth, while manufacturers pull back on fleet sales, rental companies seem to be exercising caution in their fleet buys. This discipline is always welcome, though the move is driven more by hedging against rising fleet costs in a difficult used car market.

About that used car market: Though this month’s Manheim Index is surprisingly strong, rental’s traditional segments — compact and midsize — are suffering. In June, the average price for rental risk units sold at auction was down 4% year-over-year. While rental companies are adjusting their model mixes accordingly, both Avis and Hertz expect fleet costs to rise by 4% year-over-year on lower values. J.D. Power Valuation Services expects about a 6% decline in wholesale prices for this year.

Fleet cost increases historically drive pricing. Interestingly, in Avis Budget Group’s first quarter conference call, CFO David Wyshner offered a theory on why that correlation might drive even stronger pricing today. In the past, fleet mix and the split between risk and program varied greatly by car rental company. Today, “Everyone is buying cars from the same folks in somewhat similar proportions,” Wyshner said.

Wholesale volume is still growing — a potential “used-car time bomb” of 500,000 more off-lease vehicle returns are expected in 2017 versus 2016, on top of the 500,000 more in 2016 compared to 2015. With consumer sales incentives at record highs and headwinds pushing against the used car market, a strong rate environment is welcome news. Car rental companies’ recent fleet discipline is needed now more than ever.