John C Bussey WWI Service Card

What information can I gather from my great grandfather, John Colvin Bussey's WWI Service Card?
Below is a snippet of his card.

Ancestry.com. Georgia, World War I Service Cards, 1917-1919 [database on-line]. Provo, UT, USA: Ancestry.com Operations, Inc., 2013.
Original data: Georgia Adjutant General’s Office. World War I Statements of Service Cards. Georgia State Archives, Morrow, Georgia.

What information do we find here?
1. His name.  John C. Bussey
2. His Army serial number.
3. Residence address.
4. That he was inducted.  It is my understanding that inducted in this case meant drafted and that he did not enlist.
5.  Location of induction and date.  Ft. Thomas, KY
6. Date of birth.  Nov 19, 1892  (there seems to be some inconsistency in his birthday as to if it's Nov 18th or Nov 19th, but the year is always consistent.
7. Organizations that he served in. "13 Dep Bn Sig C Franklin Cantonment Camp Meade Md to disch"
      I tried to break down what these acronyms meant.  Here is what I deciphered.
       13th Department Batallion Signal Corp.  The unit he was assigned to.
       Franklin Cantonment    The school he went to.
       Camp Meade Md
       to disch (discharge)
8. Grades, with date of appointment:  
        Pvt 1 cl Dec1/18   = Private 1st class Dec 1, 1918
       Pvt Dec 26, 1918  = Private
9. Did not serve overseas
10. Honorably discharged on demobilization Jan 20, 1919
11. 0 per cent disabled.

So I was able to gather a small bit of detailed information about the time in service for my great grandfather.  I'm now curious as to why his rank appears to have been moved down, yet he was still honorably discharged.  I have a copy of his service record, but it is very difficult to read.

I was unable to find much definitive information regarding the unit "13 Dep Bn Sig C".  I would like to find out more about what he was supposed have been learning at the signal school in Maryland.  His time in service was short, but it was something that stuck with him his whole life.  When he was older he utilized the VA for medical care.

I look forward to fleshing out more of my ancestors.  I'm currently concentrating on the military members in my direct line.