Not all tax returns are identical either, and so they move through the IRS processing system at various speeds. Other factors causing delays and increased refund processing times for many people this year are several new laws and executive orders that the IRS has been charged with implementing, including those of the 2015 PATH act.
If your tax return ever is chosen for further review, or an actual audit, then the website should indicate that, and also let you know to expect an IRS notice or letter by mail. (You haven't observed any message like that, have you?)
Taxpayers who do have their returns held for review, and who expect to receive an IRS notice, tell us that this message appears clearly on the Where's My Refund website. The behavior that you observe is more likely an indicator that your tax return is simply moving through the processing system. It could also be that the IRS is checking for additional information on something related to your tax return (like matching your reported W-2 data with that separately reported to the IRS by your employer); but we wouldn't call that a review . . . perhaps only a small delay.
If enough time passes after filing (21 days after e-file, or 6 weeks after paper file), and there is no new information, at that time the IRS indicates that you can call them, using the phone number provided on their Where's My Refund website.
For now, the best recommended course of action is patience. The IRS doesn't always move as fast as taxpayers would like (especially when waiting for a refund). Also, as noted above, the 2016 tax year may be particularly slow, due to factors that are outside of our control.
Thank you for asking this question.