Why are calculations carried out in floating-point arithmetric?

[yaccos]

I have noticed some strange computations in rowDiffs.c. In this line:

 ncol_ans = (R_xlen_t)((double)ncols - ((double)diff*(double)lagg));

all of the operands (which are of type R_xlen_t) are converted to double before the compuation and then the answer is converted back to R_xlen_t. I do not see any obvious reason why this could be computed in integer arithmetric and there is no comment explaining why this is done with floating-point numbers.