I came across an issue with a new tacho (late 2013 reg vehicle) where it was jumping the driving time forward in 2 minute increments… say you’re in fairly slow moving traffic and moving for 10-20-30 seconds, then stopped for 30-40 seconds… rinse and repeat a few times… when the next minute ticked over it wasn’t adding a minute to driving time, but it wasn’t adding it to other work either… it’s like it was saving it up and when the tacho ticked over a second minute, it was increasing the driving time by 2 minutes 
It’s as if multiple short shunts breaks the ‘longest activity in that minute’ rule, and it just decided to allocate it all to driving time. I remember this happened recently where i was stuck in crappy city centre traffic, i had about 25 minutes left to get about 5-6 miles. I must have driven for a total of about 4-5 minutes over a 10 minute period about a mile or 2, sat stationary the rest of the time… but it added 10 minutes driving on - in 2 minute increments! Still don’t fully understand how/why it’s doing that, but im wary of it now when tight on time to get to next destination, and stuck in stop-start traffic.