Probability in mathematics is just a number between zero and one. For example, we say, "The probability of this happening is fifty percent."
However, in many statements, we use "chances" and "odds", which are plural as opposed to the singular "probability". For example, consider "What are the chances of that!" and "The odds of winning the lottery are minuscule."
Dictionary definitions just tell it how it is, but not why:
You refer to how likely something is to happen as the odds that it will happen.
The following structure reads especially clunky partly because it has are next to one.
The overall odds of winning a lottery prize are 1 in 13.
— CED (emphasis mine)
Again, that last part is just one number (~0.08), so we would say, "The overall probability of winning the lottery is point zero eight." Yet, we use the plural "chances" and "odds" though they refer to one number. "What is the chance of that!" and "The odd of winning the lottery is minuscule" come more intuitively to me.