Understanding deductions with real numbers

I elected to do my own taxes this year with TurboTax rather than paying someone else because I wanted to understand how they worked.  Can someone please explain the following 2 situations:

My current estimated refund (as indicated by TurboTax is $5318.

I add an expense of $1000 to Travel Expenses.  New refund amount = $5551 (+$233)

I delete that item and instead add the $1000 expense to Meals 50%. Refund = $5427 (+$109)

Question 1: Why is the refund amount not a 50% difference?  

Situation 2: My current estimated refund is $5346 
I add the same $1000 expense to Travel.  New refund amount = $5553 (+$207)

 

Question 2: Why did I receive +$233 the first time and only +$207 the second time (a $26 difference)?

I figured there would be a standard % you get back from any expenses, say about 20%, so I should have expected the same amount for the same expense each time?

These are probably very novice questions but I appreciate anyone who has the time to help me understand!!