A Plan
The first would be that having one blessing without an extra slot means you have a harder time gaining more blessings. Yes, one could have more than one blessing, and that was the top secret the angels hid from us.
The manual was written by them, so they would never reveal such important things to us.
Many people wouldn't perform well in the quests. It was, after all, depending on many variables, attributes included.
But on the other hand, we had a good number of humans who would have a chance to gain more than one god blessing. So if they listened to these lies, and most would, they would lose a valuable chance here.
The second lie was a simple knowledge about gods; they weren't just one category. Gods, in fact, was a general term describing anyone who had achieved a big success in his past life and became a legendary figure.