Q: Clean install el capitan from the same USB disk twice, by two times have different procedure, I would like to know why

Dear all,

Few days ago, I made a USB el capitan installation disk, using the method of the Apple website.

Then I use this USB disk to make a clean install of el capitan. When my macbook reboot, I press the option key, then erase my disk, click install. The first procedure was preparing to install, which was as following picture (I did not capture the picture when I made installation,so I use a picture from the internet):

After the preparing, the procedure went to next ,which was showed as follow (also I used a picture from the internet):

Then the installation was finished.

Yesterday , because my OS X system had some problem, I decided to use the same USB disk to install the el capitan again. Just like the first time, I restart my MACBOOK, press the option key. The screen was different with the first time I made the installation. The beginning screen was as follow(also I get the picture from internet):

Then I click continue , the procedure was as follow, without the "preparing to install " procedure, the procedure is "install on hard disk",   which was as follow( also I got the picture from internet):

Then the installation was finished.

So I would like to know, I use the same USB disk to install el capiton twice, why the procedure are totally different? Thanks!!