TT is calculating the WRONG amount for the max 401(k) contribution. What's going on?

I have W2 and 1099 income. 

 

For W2, I contributed $9K to my employer's 401(k) plan. 

 

So, I want TT to calculate the max I contribute to my self-employment (1099 income) solo 401(k) plan. The TOTAL (W2 job + 1099 job) max should be $19,500, right? 

 

But I check the box to have TT calculate the max 401(k) for my self-employment business and it says $28K. 

 

What is going on???  I'm guessing the correct figure should be $19,500 - $9K (for the W2 job). So, $10,500.  

 

Does TT have a bug? 

(Curious where I can find the worksheet where TT is doing this crazy math)