If I recall correctly, Zwift made a change maybe 6-12 months ago where wattage targets for workouts were rounded to the nearest 5 watts, at least in the intervals list on the left.
But I think the actual wattage targets didn’t change. Which leads to situations like this:
My actual wattage target was 278. But Zwift displayed it was 280. Gotta say, it kind of drives me crazy, because I’m in ERG mode and can’t get the trainer to hit 280 for more than about 1 second at a time. 
I would suggest making the numbers match. My preference is to have them be exact (so 278) but if Zwift likes the “simpler” look of rounded numbers, then change my actual target to that rounded number too please!
I’ve never understood why Zwift rounds to the nearest 5 watts. I’d prefer Zwift use the plan’s specified value without rounding up or down.
The weird thing, as far as I can tell, the actual wattage targets which Zwift sends to your smart trainer aren’t rounded. It’s just the displayed targets that are rounded!
At least, that’s what I assume, since my power will stay firmly at the actual number (278 in my example above) and not the rounded one (280).
Another of my pet peeves with Zwift workouts. I would prefer to see the actual target number unrounded.
For me (on IOS) the targets do seem to round to the nearest 5 watts. It’s irritating. My power output is low enough that this affects my workouts a fair bit. I wonder why the rounding was implemented in the first place.
