51% includes also respiration of cows, which should be net zero, because it is exactly the CO₂ that was captured by the plants before. (which is not okay to do)

Also 51% because equivalent calculations are not done over the span of 100 years, but shorter (methane becomes more dominant) and deforestation is also accounted for - (which is both fine).

So it is somewhere in between the results of renovated studies that say it is around 31% (e.g. Tukker et al) and the 51% worldwatch report.