Why run-with-timer generates so many timers?

["numbchild@gmail.com" <numbchild@gmail.com>]

I write an extension rainbow-fart.el (
https://github.com/stardiviner/emacs-rainbow-fart)

Here is the code lines which running timer:
https://github.com/stardiviner/emacs-rainbow-fart/blob/master/rainbow-fart.el#L245

When I run command `list-timers`. I found many timers is generated by
rainbow-fart.el

#+begin_example
275.87      900.0    rainbow-fart--timing-remind
278.20      900.0    rainbow-fart--timing-remind
280.55      900.0    rainbow-fart--timing-remind
280.65      900.0    rainbow-fart--timing-remind
280.66      900.0    rainbow-fart--timing-remind
280.74      900.0    rainbow-fart--timing-remind
280.86      900.0    rainbow-fart--timing-remind
280.94      900.0    rainbow-fart--timing-remind
281.14      900.0    rainbow-fart--timing-remind
281.19      900.0    rainbow-fart--timing-remind
281.21      900.0    rainbow-fart--timing-remind
281.28      900.0    rainbow-fart--timing-remind
281.81      900.0    rainbow-fart--timing-remind
281.89      900.0    rainbow-fart--timing-remind
283.31      900.0    rainbow-fart--timing-remind
283.39      900.0    rainbow-fart--timing-remind
285.49      900.0    rainbow-fart--timing-remind
.....
#+end_example

Is my `run-with-timer` source code wrong? Or did I misunderstand this
function?

Hope experts like you can explain it for me, thanks very much.

[stardiviner]           <Hack this world!>      GPG key ID: 47C32433
IRC(freeenode): stardiviner                     Twitter:  @numbchild
Key fingerprint = 9BAA 92BC CDDD B9EF 3B36  CB99 B8C4 B8E5 47C3 2433
Blog: http://stardiviner.github.io/