If one visually inspects the table (it's not really a graph) vertically from the x axis (the horizontal) it's easy to see that for a given temperature the heat index decreases as RH % decreases. That is driven by the model and has nothing to do with the table not providing RH % values outside those typical in Tucson's pre-monsoon summer.
From safety and comfort perspectives, ceteris paribus, it's probably better to cycle in the Old Pueblo when it's hot and dry rather than when it's hot and humid (monsoon) Wicking effect is better when RH is low. Either way, hydrate appropriately. And learn about UVA and UVB, melanoma (typically starts slowly in childhood), and sunscreen. Learn the early signs of heat exhaustion and what to do.