Var types and math; unexpected results?

I'm generating a seeded "random" value wit$$anonymous$$n a shader, along an arbitrary grid size. For example, every 50 units can be deemed the grid size, and every 50 unit range has an identical psuedorandom return value wit$$anonymous$$n that range.

 float multiple = 50;
 float grid = floor(vertexPosition.x/multiple);
 //grid is now the multiple, for example, x position 120 = grid 2
 float p = distance(vertexPosition.x, grid*multiple) / multiple;
 float distortion = lerp(psuedorand(grid), psuedorand(grid+1), p);

Problem is, p is returning unexpected values (over 1), because the distance between vertexPosition.x and grid*multiple is coming out larger than 50. I can't figure out why t$$anonymous$$s would happen. Grid*multiple must surely equal where the 50-unit space that the vertex resides in begins?