Look at the color book you pulled it from. If the swatch is on glossy cardstock, it is TPX. If it is on soft, fuzzy cotton, it is TCX. If you don't have the book, assume it is TCX (the modern standard).
: Printed on paper; used for hard surfaces like accessories or home goods. pantone tcx to tpx converter
A: Because monitors use RGB light. Pantone TCX and TPX are physical dyes/pigments. You cannot "convert" dry pigment to light perfectly. Look at the color book you pulled it from