EV100 is just a widely accepted constant for measured values at ISO 100. You can do the math to adjust for different ISOs, but I think it’s safe to assume that most EV charts online will be based on an ISO of 100. There’s probably a particular reason for film, but only thing I can think of is it’s generally more suitable for a wide range of scenes with little visual issues, compared to high ISOs getting grainy. In games I don’t think it matters as much since you can change the f-stop and shutter speed for any ISO and get clean results.
The values are technically the same, they’re just presented differently. An EV of 5 is identical to a shutter speed of 1/4s with an f-stop of 2.8 at ISO 100. It’s also identical to a shutter speed of 1s, ISO 100, and a f-stop of 5.6 because both combinations of settings will result in identical exposure. EV5 is just a simpler way to put it.
And that’s where your settings messed up, you aren’t using the right f-stop for an EV of 8 with the shutter speed at 1/125s. Try 1.4. You can use a chart like this Wikipedia one to narrow down the individual camera settings for ‘manual mode.’ Exposure value - Wikipedia