Broadly speaking the hybridization scheme is similar between ammonia and phosphane. However phosophorous is a bigger, more diffuse atom than nitrogen. Because the bonds surrounding nitrogen are closer together, they exhibit more repulsion between each other, which increases the bond angles. In phosphorous, the bonds are naturally farther apart and so the bond angle can be shrunk in order to reduce interactions with the lone pair. In phosphorous the lone pair can be described as being located in a more "pure" unhybridized atomic orbital. But this is a spectrum of possibilities, not an "either/or" proposition. Some degree of hybridization can be invoked in both cases to explain the structure.