I've been told this in the past, and in my experience I'm going to disagree with the others here and say you are being fed BS. Basically, they have no intention of keeping you, but they are keeping you on staff for as long as they can until they can find someone to replace you; once that's done, you're out the door. Your company probably moves in hiring "waves", where they hired you and a bunch of other people at the same time but then they didn't hire anyone else for a while. Your job is probably already posted as part of the next hiring wave, and they are just waiting until they find someone to replace you.
As for why they lied, they did it so that your morale would be higher. Nobody wants to hear "you're fired, but we want you to stay until we can find your replacement", because your morale will be low, your sense of ownership of your tasks will be low, and your productivity will be low; in extreme cases you may even actively sabotage others because you have "nothing to lose". Promising that you might have a chance to continue staying makes you think that you can impress your way out of the situation so you will work harder (or at least as hard) as you did before and be a compliant employee while waiting to be fired. The "trial week" means that they have already interviewed someone and are about to send them out documents; it takes about a week of turnover time between when the documents are sent and when they are signed and received, so they don't want to be short-staffed for that week, that's why they've extended your stay by a week rather than terminating you on the spot. Realistically speaking, a week is not enough to change the impact of a month's worth of habitry; they don't expect that and neither should you.
Remember: HR is not your friend. They are not there to be your advocate, or liaison, or anything else. HR will do whatever is best for the company, and if lying to you is best for the company, then they will lie to you.