Hi,
I also have been using a Dakota 20 with the HR strap.
#1 - How much of an error are you seeing? In general it is not going to match up completely, but should be close, especially if you are recording points every second.
#2 - Are you recording one point per second? If not, the unit is most likely averaging the values, which would mute your actual max. I've been paying attention on my rides lately and have so far registered the max I've seen.
Hope that helps.