Why does this image look sharper in flash than HTML5?

Hi I'm trying to help a colleague with a project.

It's been built in Captivate 2017 and have been previewing it in flash as well has HTML5, although it will be published to HTML5. They've noticed that some of the images in HTML5 preview look either fuzzy or less sharp then the same images in flash. I find this really weird as in the past I've usually found projects look sharper in HTML5 than when published in flash.

I thought it was something unique to cap 2017 as I've been using version 9 up until recently. But I tested it in version 9 and cap the same thing. I've attached both pictures so you can see the difference - you can see it on the detail of the people and also on the button on the left. It's jagged in the HTML5 version but sharp in flash.

It's not been set up in the ideal way. The image of the two people for example is actually  approx 2000 x 2000 px but has been scaled down in Captivate to 300 x 300. Usually I'd import any images at the same res they're going to be in the project, however it's a colleague who has set this up.

I did try exporting the image, resizing it and bring it back in at 300 x 300 but it then looked like the HTML5 image in both previews. I also tried publishing them to see if it was preview being weird but got the same results.

Is it down to the way HTML5 and Flash files handle images/are rendered? If so what's the difference that has ended up with it looking sharper in flash?