a = {{1, 2}, {3, 4}, {5, 6}} ;
FromDigits /@ Transpose@ a == FromDigits @ a
(* True *)
The only "strange" syntax I've found in the docs is
But it doesn't seem related to this behavior. Or at least I don't see how.
Note that when there are only two lists, the second one is considered as base 10 "exponents" and threaded upon.
FromDigits @ {{1, 2}, {3, 4}}
(* {120, 1200} *)
that comes from the syntax above
FromDigits @ {{1, 2}, 3}
(* 120 *)
{{1,2},{3,4},{5,6}}
becomes100*{1,2}+10*{3,4}+1*{5,6}
, that is, each sublist is treated as a "digit". $\endgroup$ – Daniel Lichtblau Sep 18 '15 at 14:34