The display of the BPM requires a smoothed out calculation of the past time between each clock. This calculation can be fairly processor intensive depending on how you do it (using floats typically, so it takes more processor time). So a difference of .5bpm is pretty reasonable on a display
The actual response of the instrument will not be affected as the BPM is responding directly to the incoming clock, not the number it's displaying on the screen.
When the Tempest is the master, all other devices in the room show continually fluctuating BPMs.
When a Rytm (or many other devices) is inserted into the same set up in place of the Tempest, all other devices match the BPM exactly and do not fluctuate.
This tells me that the culprit is the Tempest, and not the processing capabilities of every other device in the room. As I said earlier, everything stays in sync, it's just the slaved display of BPM of all other devices that is the problem.
What is it about the Tempest that would cause this to happen?
I honestly can't believe we're still hearing the "every clock drifts, and tempo is calculated thusly" excuses. For the last time, so help me the powers that be, the Tempest's clock runs slow. Period. In fact, at a tempo of 78 BPM, it lags by nearly a 10th of a BPM. Which is why every single device that is slaved to the Tempest shows a discrepancy in tempo, just as Stoss observed above.
Folks, I've been doing this a long time. Music is my job. For 30 years I've been paying the bills as a musician, producer, and audio engineer. The last time this controversy came up, my esteemed colleagues and I decided to gathered together every drum machine and groovebox we owned: everything from a lowly Alesis SR-18 and a Korg Electribe 2, to an entire array of high-end music production stations covering every Elektron box ever made, a few Roland machines, and several models of MPC including the new MPC Live... Nearly two dozen boxes in total. We even threw a few iPad apps in the mix just for fun. First, we ran them all as slaves to the Tempest, and predictably, they all showed the same discrepancy in BPM. Then, just for shit's and giggles, we ran them all unsynced. We pressed play on each box, by ear, with no MIDI cables hooked up whatsoever, taking care to make sure they all started in sync, old school...
The result was undeniable: The Tempest fell out of sync within the first 8 bars. I kid you not. And I mean jarringly out of sync! No matter how many times we restarted it, the results were the same. Meanwhile, the other boxes stayed notably and useably in sync with each other for miles by comparison. The worst of them made it 10 minutes or so before the trough got too wide, whereas we were still jamming with the better contenders nearly 40 minutes later, having all but forgotten we weren't actually hard synced.
Take from that what you will. But that's the truth. Go ahead, if you've got a few different machines kicking around, try it for yourself. You'll see.
Cheers!