Code:
Block : 335
Blocktype : 1 (Fixed trees)
Offset : 12,555,965
Size (unc) : 9,251 bytes
Size (unc) : 74,008 bits
Bits (com) : 4,051 (18.27 bits/bit)
Bits stored: 74,047
Block : 336
Blocktype : 2 (Dynamic trees; L-codes: 286; D-codes: 30)
Offset : 12,565,216
Size (unc) : 35,645 bytes
Size (unc) : 285,160 bits
Bits (com) : 5,703 (50.00 bits/bit)
18 C-codes (54 bits); 690 bits for trees; 761 bits total (95.125 bytes)
Bits (new) : 5,703
18 C-codes (54 bits); 690 bits for trees; 761 bits total (95.125 bytes)
Bits fixed : 6,563
Bits stored: 285,196
Bits (redo): 5,703
18 C-codes (54 bits); 690 bits for trees; 761 bits total (95.125 bytes)
In verbose mode it prints stuff like that.
So imho it tries to optimize the compression of deflate block headers.