I have a brew session where the numbers don't make sense to me.

Here's what I think is the relevant info from the recipe (it's BIAB if that matters):

- Efficiency: 70% (ending kettle)
- "Starting boil volume" under Water Requirements: 2.88 gal (it's for a 2.5 gal batch)
- "Recipe based estimate" of boil gravity: 1.048

I thought that boil gravity would be same as "pre-boil gravity" in the Brew Log, so I expected that if I collected 2.88 gal from the mash, and if my pre-boil gravity was 1.048 or better, then my pre-boil efficiency in the log would be at least 70%. So, I was surprised when I recorded the following in the log:

- Pre-boil volume: 2.9 gal (i.e. essentially identical to what was expected)
- Pre-boil gravity: 1.051 (i.e. more concentrated than expected)
- Calculated pre-boil efficiency: 67% (i.e. less than recipe efficiency)

I know that pre-boil efficiency is not "the same" as ending kettle efficiency, but the FAQ says Ending Kettle Efficiency is:

*Theoretically the same as Pre-Boil Efficiency, provided accurate measurements at both points (minus rounding error)*

What am I missing or misunderstanding? Thanks in advance for any help understanding this.