You will need to do X number of hashes to solve a difficulty 1 block, that number X will vary. So the time it takes for you to submit a diff 1 share will vary from share to share.
The same is true for a diff 8 share, except that there will be a greater amount of variance due to the greater difficulty.
When you calculate submitted shares that is looking back over what has been submitted, and so a calculation of the equivalent value will of course match.
see those stats above? that is direct from the cgminer program to this post. They are exactly the same in other words , for however many accepted difficulty 1 shares you get it will be divided by the pool difficulty and that becomes your accpeted shares---in other words --quit the mining program before getting a accepted share will lose all difficulty shares even if there was enough for a accepted share before cgminer did its scan log.