what to believe

id go with half of what the bike computer says. i hate the labels of light or hard effort because its different for everyone. to find out exactly how many calories you have burned you'll need something like a Fitbit, but even then its an approximation.

this is why i dont bother with exercise calories, i just drink a 250 cal protein shake after exercise and call it even no matter what

200 cals for a half hour of biking seems reasonable though, especially on the highest tension setting

That doesn't sound entirely unreasonable - if you're a 200-lb young man, 400 calories in half an hour would be half an hour of really hard work, but possible. 200 is probably too low, unless you're pedaling so slowly that the thing barely registers.

If you really want to know how many calories you're burning you need a Heart Rate Monitor, they're coming down in price a lot these days although I would recommend something from a reputable brand.

Why do you need one? well as you say, on a stationary bike speed is nothing and neither is distance - it's about resistance and exactly how much effort you put in. Without any other data to go on it's hard to tell if the estimation is correct, I know on my bike I'd burn 600-750 cals an hour but that's with an HRM used indoors and out so I can compare my indoors activities to those out on the road or trail to ensure consistency in my training.

I'd go with the 200 cals, it's the best estimation you've got so far.