The intent must be right. 13t is pulling 155mm M1 Howitzer and 18t is pulling a heavier M1 8-inch Howitzer or a Long Tom, it's just that the designations are flipped. And I also didn't report it quite right - the heavier prime mover and guns are in Greyhound Dash (not Manhay Crossroads)
The designations (M5 Tractor/M4 Tractor) are correct, but their classifications ("18t" and "13t") are flipped.
In the best case, this is just a typo.
What I wonder about, what ranges and what max speeds were specified for these units? While the trucks and jeeps were relatively fast, the tractors were not supposed to go faster than 35 mph. For the bigger guns (240mm?) M6 Tractors were planned to be the prime movers, but the production was hampered, so modified M3Lee tanks and their tank retriever versions were used to pull those big pieces for quite a while.
For long distance redeployments, I could imagine that the Tractors were put on flatbed trucks/trailers, as these beasts were real gas-guzzlers, the M4 consumed 2 gallons per mile when it towed an 8-inch howitzer. That reduced its range (180 miles @ cruise speed) to 90 miles (with towed load). Imho, the max speeds of their slowest elements (the tractors) and the actual ranges of the tow-equipment (with towed loads) should determine the average speed and the max. ranges of such units, unless there's evidence that they were actually pulled by tanks or put on trailers for longer distances. For the latter case, the prime movers should be removed and replaced by tanks/flatbed trailers and their respective max speeds, as the tractors then would have been used for short legs (tactical relocation to avoid counter-battery fire) and slight adjustments of the guns/the fire base, only.