If plus means, all the light rays are focusing behind your retina
& if minus, it's in front of the retina, in both horizontal & vertical meridian.
But if you have cylinder power, then both the vertical and horizontal rays are not focusing together.
Therefore, when you have a cylinder power in addition to spherical, it means all a portion of rays are focusing behind & another bunch of rays focusing in front of the retina.
So +1Dsph/-2D cyl at... is different from just +1D
Hope I helped you....