Question : Help with understanding DNS propogation

I recently setup a domain in Google Apps and once the domain verification process was complete, I changed the MX records from the existing mail host to those for Google Apps.

I ran a number of tests on DNS Stuff, as well as a few domain propagation websites, and all pretty much immediately returned the updated (Google Apps) MX records in the results.

Here's my question: I'm used to waiting a few hours for domain DNS changes to propagate, but was surprised about how quickly the updates were returned in my tests.  Furthermore, 6-7 hours later, the old email account is still receiving mail, and nothing has come through Google Apps.

There's obviously a gap in my understanding of DNS propagation here...

Thanks in advance!

Answer : Help with understanding DNS propogation

At the end what is occuring is that all DNS's aournd the world has to get those changes and that is why you may get email continue to go to the old account for a while.  What might happen is that you will fine some will go to the old and other will go to the new.  I all has to do will what DNS server the person who is sending the email is going through.  When you make a change the give a time line of up to 72 hours for certain changes.  Give you an example of what could happen within that period of time.  Let's say you have two service providers in your city.  One person with provider A, lets say 3 or 4 hours after your change,  could email and it works and goes to you new account settings; the other person in the same city same time but with provider B, sent the email and it does go to you new account setting .  the reason for this is that those provider have ther own DNS servers and one provider has recieved the changes in their DNS and the other one has not.