JMODEM applied RLE on a block-by-block basis, as opposed to the file as a whole. Since many files were already compressed with systems like .zip, JMODEM only used RLE on blocks where it actually reduced the size of the block.
JMODEM employed RLE compression on a block-by-block basis, rather than the entire file.  Because many files were already compressed with tools like .zip, JMODEM only used RLE on blocks that resulted in a smaller file size.