After thinking about changes to the length of a day on any planet, I am inclined to suspect the measuring system in use rather than any change in the rotational velocity of a body as huge as a planet. I mean, you are talking about some seriously huge forces that need to come into play when discussing a change to the inertial forces involved. But errors introduced while setting up and operating the measuring system could easily produce data that is a tiny bit random.
JMHO ...
The earth's gravitational pull on Venus must be responsible in some way for that relationship. I read somewhere that Venus always turns the same side to the earth (like the moon does) but my math skills aren't good enough to be able to verify that.