The ranking will be based on the following:
Keyword bid- keyword bid is one of the factor that will make your ads more competitive. You can change it anytime you and and so your competitors, that's why the rank changes from time to time.
Quality score- quality score makes your ad more competitive. High relevance of the keywords that you are using going to your ads, then going to your website will make your quality score higher and it will become more competitive and will give a higher rank to most relevant searches. Quality score will also based on the number of impressions and the number of clicks. If you have 1 click and 100 impressions for example, it will make your ad less competitive as it lower it's quality score if most of the impressions did not turn out to clicks. That is why it is highly recommended to use keywords that are highly relevant to your website.
You can also know more about it on this from Bing: