Okay,

trying to make a long story short, too late, I know.

I can do 30 minutes on an elliptical and my fitbit will tell me I have 25 minutes or so of "zone fitness" but I do 30 minutes on a Concept 2 rower, and I get 11 minutes, even though I'm wiped at the end.

Just a failure of the Fitbit to be able to register it?  Or do I need to actually increase the intensity on the rower to get it to kick in?  For those of you who time things or are familiar with the devices, the elliptical is a Precor and is on resistance 10-12 with an average of 2.5 miles per 30 minutes, and the Concept 2 Rower is on the 5 setting and 30 minutes was a 6000m distance.

It's probably basing it of your average heart rate.

I would assume that the elliptical keeps you at a lower but more steady heart rate like you would get from hiking or running.

The rower is likely more intense and pushing you out of that "ideal" zone that your app is looking for.

HR zones as a measure of relative intensity depend on exercise mode.  Why?  Because HR is a rough measure of O2 use and the more work you are doing the more O2 you are going to use.  As a general rule, you will be able to do more work with your lower body than with your upper body and while an exercise mode like XC skiing that uses primarily lower body muscle mass with a small assist and balance work from the upper body will result in higher HRs for moderately trained XC skiers compared to runners, I suspect that rowing will not allow you to sustain the same level of work for an extended duration.

The upper body contribution to rowing is substantial and occurs in series with the lower body contribution.

Additionally, on an elliptical you are upright and your heart has extra work to do against gravity that it does not on a rower.

In triathlon, athletes can generally sustain 5-10 bpm higher HRs running than cycling and much more than that above swimming HRs.

Bottom line, those zones are generally organized around running HRs and, if you want to use HR as a measure of intensity, you should have a separate set of zones for each exercise mode.  

I don't think it's worth it and I abandoned HR training in the late 90s because of its limitations.