An assumption would be a supposition that something, which isn't directly proven by the known facts, happened and led to the event you're trying to explain. Example: fire starts outside your house in drought season. Possible explanation #1: someone threw a glass bottle which caused a fire to start in the dry grass. Explanation #2: Your neighbor had a grudge against you and doused the grass with gasoline to do arson.
The first explanation makes one assumption: we don't know if someone did litter or not, it's not an unreasonable thing to guess but it's also not directly related to the facts we already know. The second explanation makes several assumptions: That our neighbor had a grudge, that said grudge drove them to use gasoline specifically to douse the grass and burn it. These things may be true, but none of the facts relevant to the situation directly prove either of the assumptions.
An assumption would be a supposition that something, which isn't directly proven by the known facts, happened and led to the event you're trying to explain. Example: fire starts outside your house in drought season. Possible explanation #1: someone threw a glass bottle which caused a fire to start in the dry grass. Explanation #2: Your neighbor had a grudge against you and doused the grass with gasoline to do arson.
The first explanation makes one assumption: we don't know if someone did litter or not, it's not an unreasonable thing to guess but it's also not directly related to the facts we already know. The second explanation makes several assumptions: That our neighbor had a grudge, that said grudge drove them to use gasoline specifically to douse the grass and burn it. These things may be true, but none of the facts relevant to the situation directly prove either of the assumptions.