Everything with a lithium battery has this operation to prevent the batteries catching on fire - literally.
The old battery tech would trickle-charge once at 100% and if you were using it while plugged in, the actual power to run would come from the charging circuit.
The new stuff - including your Evo - charges to 100% and then immediately starts running on battery. When it hits its low point, 92~93% on the Evo, it starts charging again - other devices hit theirs at the 95~96% mark - it's make/model dependent.
If you take your phone off the charger the moment it hits green, assuming a non-damaged phone/battery, it will show 100% and not drop quickly - because it's really at 100%.
If you wait and take it off, it'll show 100%, but really be lower and it takes several minutes for the actual power levels to update.
This gives the appearance of rapid battery drop or failure to charge to full power - but that's not the case. Laptops behave in exactly this same way - except people often buy laptop batteries unnecessarily over this misunderstanding.
If you were to magically be able to graph anything with a lithium battery charging, it would kinda trace out what looks like a rollercoaster - up to 100%, down to the low set-point, up to 100%, down to the low set-point and so forth.
How fast it drops on the charger depends on what processes you may have running and updating overnight - that will vary for each user.
As for the Shift specifically - it's going to behave the same way by definition, but it's not as noticeable. The components in the Shift are just way more energy efficient than the Evo - and it's sporting the Evo's battery. So - it's already going to rollercoaster way less than the Evo - and I suspect that they've raised the set-point to 95% or above - because with this silicon, they can.
Hope I made a lick of sense somewhere there.
