I'm not mixing them up, it's just that your examples are very vague.
You need to contextualise "disadvantage" because how you've presented it leaves much to the imagination. For example, one runner is faster than another, that is their advantage, the other run is disadvantaged because of this. To level the advantage wouldn't be fair.
Disadvantaged can mean a myriad of difference things but at a base level.
In this case having a lower chance to succeed due to external factors outside of their control, be that growing up in poverty, a lack of positive role models, lower intelligence (as a result of poor nutrition in early childhood), lack of teaching of temporal discounting, substandard schooling, abuse (physical, emotional, sexual or neglect) in childhood, excessive bullying during adolescence, familial criminality/substance abuse/alcoholism, disrupted family life (divorce parents at early age - messy divorce).
All these factors & many more will have an impact on a persons chance to succeed, I just favour a system which accepts at a high level that there is no such thing as an equal start & therefore ensures everybody enjoys at least a reasonable standard of living off the back of this.
Regarding the runner example, really genetic advantage does make the matches unfair - but the 100 meters isn't meant to be a 'fair' race, its' meant to see who can run the fastest (this distinction is important). While having the same training opportunities is also another factor (which could also be a counter) overall they are not comparable as life isn't a race to see who can run the fastest.
In society we wrap up a rigged race as being a shining example of a meritocratic one & then berate & insult the loser. In this 'race' we judge those who we think are 'trying to run' (ignoring that some may have crippling physical aliments which prevent it)