I am not saying there's bias, but for the athletes, they are in their comfort zone, so maybe, they are performing much better.
Also, they have the large part of audience supporting them giving a good boost in their morale.
As you pointed out and if we refer to the history of SEAGAMES, host countries do have a 90% chance to dominate the overall medal tally.
I remember when our country hosts the event, the crowd is really intimidating and those cheers will really pump up the athletes while on the other hand, it's a big distraction to the visitors. That alone is a hard wall to break for a visiting country.
And no, there are no biased results in most cases as games are witnessed live and as far as the overall experience is concerned, no such thing as several big controversies happening yet up to now.