It would be my guess, they are not intentionally attempting to 'fool' potential employees. They are ensuring you know your stuff. Regardless of whatever their need for you is right now, they want to ensure that if they are employing a coder then that is what they get. If all they needed at this moment was a web designer (please ... all web designers... I know your job is not 'easy' don't kill me for this next line of thought), then what would happen if they interviewed someone with the stance that "right now we need an HTML 5 web site designer" so right now that's what we'll get?
For the job necessary at the moment, they will get exactly that. Once the page is up and it looks good and everything works, well they're not going to be forever satisfied. So, now they need some PHP code, perhaps a java module. Maybe some Perl coding as well. They go to their newly hired HTML 5 expert and say we need this and that, and the employee now says OK, I'll need 6 months and four classes to learn it to get up to speed.
The company can spend the time and training money and advance the knowledge of the employee, for sure. They can also fire the employee and hire a new person with the required skills. (Depending on where this is happening, it may be more or less difficult mind you.). I am in Virginia, USA. This is an at-will employment state. The company could let the person go, because they no longer meet the skills required, and there would be little to no legal ramifications, beyond perhaps unemployment.
They could hire a person with the exact skill-sets needed plus all the additional knowledge needed to stay current for the foreseeable future as well.
So what should you do now
Ask for a meeting with your manager/boss. Be honest and up front with him or her. Let them know that after the battery of testing that occurred during the hiring process you thought X while the reality is Y. You are currently kind of unhappy with the situation, as you feel you will quickly lose skills. Ask him/her what the plans are both near-term and long-term, for the software, the project and the company.
This will show you have a concern about your own skills, but also that you are thinking to the future, both of yourself AND the company. For all you know, there may be projects just around the corner which will amaze you.
After that meeting, see how you feel. If you are still very much concerned, then re-evaluate your own career goals and hey, maybe it's time to refresh your CV and continue the job search. If that is true, you have one advantage over many other people right now.... you already have a job!