Most Helpful Girl
The reason that sometimes it "matches" your day is because the "predictions" can be so vague on purpose to try to cover as many people as possible (like Virgo, for instance, is like August 22-Sept21 I believe- that's a lot of birthdays and Virgos to cover).
Also, confirmation bias. You're going to purposely try to match the situation with the vague description.
Your horoscope says "you will have good fortune today". And then your direct deposit hits your bank account or it's payday, and you think the "prediction" was correct when really you were scheduled to get paid regardless.
And sometimes the vague predictions are wrong that's why you think it didn't come true.
Most Helpful Guy
There's a science behind this "cold reading" kind of stuff -- it boils down to being vague enough to seem accurate with a wide range of people, while simultaneously making them convinced that it's specifically targeted to them.
It's all trickery -- but entertaining tricky at times since it teaches us a bit about human nature. We also tend to be conditioned to pay attention to hits while ignoring misses.