This seems to be a difference in rating criteria. S&P or Moody's rate the GNMA fund AAA, which means extremely low likelihood of default. The "B" rated corporate debt means higher likelihood of default. S&P and Moody's assign their ratings based on their evaluation of the issuers ability to pay.
You didn't mention who assigned the "1" or "2" ratings (Vanguard?). My guess is this is likely based on different criteria than what S&P and Moody's would use, perhaps past performance of the fund, volatility, and other criteria similar to what Morningstar would use. It's worth reading the fine print on what the ratings are based on, but this seems to be a difference between:
1. rating the funds and their performance and volatility, versus
2. rating the issuers of the bonds and their ability to repay
Again, I would dive into the details before investing, but there is probably a tradeoff like: the GNMA has less likelihood of default loss, but the Short-term bond fund has less likelihood of volatile downward movements.