Tuning Question For Gurus! URGENT!!

Tuning Question for Gurus!
We have a stored procedure which has the following "where" condition a,b and c are same table joined to itself.
This was taking 1 hour to run previously(which caused severe blocking on the server).Then, I modified the "where" condition as follows to improve the performance:

OLD CODE(where condition):
---------------------------
Where getdate() >= a.Start_DT
AND getdate() >= b.Start_DT
AND getdate() >= c.Start_DT
AND(getdate() <= a.end_DT OR a.end_DT IS NULL)
AND(getdate() <= b.end_DT OR b.end_DT IS NULL)
AND(getdate() <= c.end_DT OR c.end_DT IS NULL)

MODIFIED CODE(where condition):
------------------------------
Where getdate() >= a.Start_DT
AND(getdate() <= a.end_DT OR a.end_DT IS NULL)
AND getdate() >= b.Start_DT
AND(getdate() <= b.end_DT OR b.end_DT IS NULL)
AND getdate() >= c.Start_DT
AND(getdate() <= c.end_DT OR c.end_DT IS NULL)

This new code has improved the performance,and now the query is running in 1 sec!

BUT now the big question is:
------------------------
All of a sudden the old stored procedure is also started running in 1 sec! Any clue on why it happened? Also,which code is superior, performance wise??
I need to explain the reason to my management,as this has been a major problem all these days.Also,it will help them to put the right code in production.
Thanks!!
sa.