Two reasons:

1. Your last 30-60 days is included in this ration; in the last 30-60 days cattle gains do slow down because the cattle are laying down more fat and less muscle, and typically intakes drop as the cattle finish.

2. Even though the gain and intakes do trail off towards the end, DNS (the software used here) tends to overestimate this drop off in gain and DMI. The intakes and efficiency can be set by period which allows the software user to make these numbers more real and accurate.

Even though the last period is underestimating performance here, the payweight to payweight projection above this section would still be accurate, because the software is overestimating the gains in the first and second periods.