In order to round 1 decimal place to 5.0, the average needs to be 4.95 or higher.
19 5-star + 1 4-star = 4.95
39 5-star + 1 3-star = 4.95
59 5-star + 1 2-star = 4.95
79 5-star + 1 1-star = 4.95
In order to round 2 decimal places to 5.00, the average needs to be 4.995 or higher.
190 5-star + 1 4-star = 4.995
390 5-star + 1 3-star = 4.995
590 5-star + 1 2-star = 4.995
790 5-star + 1 1-star = 4.995
So, if you got just one 1-star review, your rating won’t display as 5.00 until you have 790 5-star reviews, and in that case, well, @Debthecat’s answer is correct.