This is an effect of the unknaszero option.
Sometimes, if your graph generation is sufficiently out of synch with the data collection, then the graph generation falls into the next sample window of the RRD. This means that it returns an 'unknown' for the last value (since no data has yet been collected for this window).
routers.cgi has a workaround for this, since it usually only affects the userdefined graphs, but if you have unknaszero then unknown is already set to 0 and so you get this effect. I'm not sure how mrtg-rrd avoids its -- I suspect it doesn't, but that you just asked it to generate the graphs in the same time window as the data collection and therefore avoided the issue. With touters.cgi, due to its caching and expiry, once you get out of the window it's hard to get back in again
I'll look more into finding a more reliable workaround for this -- maybe using the lastupdate time as the base if it is more recent than the update interval... (you can set 'uselastupdate = yes' in the routers2.conf to have this affect, also)
Steve