At this point in Survivor: Winners At War, Sandra has been in the game long enough that she’s well behind others on the Edge of Extinction who have Fire Tokens.
She’s also not all that physical of a competitor, and a vast majority of what contestants have been doing on Edge of Extinction has been physically searching the island for clues.
Sandra may just realize she can either bust her tail for a snowball’s chance at hell at returning, or quit early to relax and live a normal life at the Ponderosa.
Personally, it’s something I don’t have trouble believing.