hareeb bro,

previously all the graduating students were allowed 12 months of OPT,  but recently they changed this law and So the new law states that if u are one of the STEM (Science, Technology, Engineering, Mathematics) student, you are allowed to extend your OPT for additional 17 months  which means you can get upto 29 months of OPT period and that allows you to apply more than once for H1 incase u become unlucky for the first time as is the case there have been more applicants than the allocated quota for H1...... 

but if you get your H1 on first attempt you don't need to apply for extension.  And that extension is only for those who have one of STEM degrees....