Weird! So your pension estimate assumes future employment, like Social Security. This has not generally been my experience for pension calculations. Well, that explains that difference.
Pension estimators, much like pensions themselves, can differ. E.g., Megacorp allows one to defer the pension after retiring. In return one gets a higher annual payment when payments do start - again similar to SS. Apparently (from comments in other threads) not all pensions have this feature.
In general or in this case? I think in general this is a good assumption, but I think the presence of a potential large pension means that in situations like this one, it needs to be more closely examined.
If one puts "too much" into traditional (i.e., retirement income is higher than projected) the downside is paying more taxes than necessary on the traditional withdrawals. The person still has more than enough money so quality of life is unaffected. The estate at death is a little smaller than it would have been if more Roth had been used.
If one puts "too much" into Roth (i.e., retirement income is lower than projected) the downside is not being able to reclaim the taxes that were paid upfront when the money is needed in retirement. The person does not have enough money in retirement, so the quality of life suffers.
Errors in estimating retirement income can occur due to misestimating market returns, contribution amounts, pension and/or SS income, etc.
The closer one gets to retirement, the more accurate the estimates are likely to be.