I suspect the advertisement you saw did not say you were eligible, but was simply an advertisement for a service that had a free trial. Most ads have the eligibility rules listed in the ad or seen by a link. Apple does not give out any of your purchase history or personal information to advertisers, so they have no idea if you had used a previous trial.
When you clicked the link for the free trial, were you taken to the developers web site? In this case the subscription/free trial is being directly billed to you from the developer without any involvement from Apple. The only subscriptions that are managed by Apple are the ones you make with an in-app purchase inside of the app, not on a website. The in-app purchase will show if the free trial is available along with the subscription price and start date before you double click the side button to authorize the transaction.