Warning: no knowledge of Yampa.
It seems that the integration for the position is always using the velocity from the previous step. Looking at the documentation in source code of Yampa this seems plausible as there is also a function called imIntegrate with the comment
-- "immediate" integration (using the function's value at the current time)
Which I assume would yield your answer. As I don't know Yampa I can only speculate about the reason for this difference, the two things I can think of now are better speed and to prevent trouble (in cases like x = integral y, y = integral x).