Barnabas ends at Quire 92 Folio 2.
Hermas beings at Quire 93.

The standard explanation is that some other books were placed on the other six folios of quire 92.
And these speculative and unknown books fit rather perfectly to the end of Quire 92.

(We can pull out some quotes from the Sinaiticus writers.)

David W. Daniels pointed out a more likely scenario. Hermas was done after Barnabas was completed. Perhaps in a separate location or time.

Makes sense, and this is also consistent with the fact that, while all the NT and Barnabas are scribe A, all of Hermas is scribe B2.

On top of that, in the late production scenarios, Hermas might have been given special consideration, as its development could be intertwined with producing the Hermas text source materials that gave Codex Lipsiensis.