Not all types of cotton are the same, and not all cotton sheets are woven the same way. Different terms indicate the fiber content and manufacturing process used on the sheets.
Pima is a type of cotton. The pima cotton plant is crossbred from Egyptian and American cottons. It produces extra-long fibers. The term percale refers to the way the cotton is woven; percale cotton sheets have a thread count of at least 180 threads per square inch and the same number of threads lengthwise and widthwise.
Because the term percale refers to the weave of the sheets, not all percale sheets are 100-percent cotton. Some percale sheets are blends. Pima cotton sheets are 100-percent cotton unless otherwise marked as a pima blend or a mix.
It is possible to have pima cotton percale sheets. This means the sheets are made from pima cotton but woven in a percale fashion.