The mod portal tries to highlight a diverse set of mods by avoiding mods and highlight categories which have been featured recently. It's also a heavy user of the random.sample function.
To ensure a high quality mod selection it will also not highlight mods for which one of these is true:
- Mods in the Internal category
- Mods without thumbnail
- Mods incompatible with Factorio 1.1
- Deprecated mods