My federal tax dropped low why !

There can be many reasons why your refund changes.  Keep in mind that each entry taking place in your return changes the outcome and TurboTax is calculating this with each entry so the refund or balance due will fluctuate until you have completed your tax return.

The differences from one year to the next can be a change in earnings and/or the amount of withholding by your employer compared year over year.  One of the fastest ways to tell what changes there are is to compare line-by-line last year's return to this year.  

By using the Tax Tools you can take a quick review of your 1040:

• To find Tools: 
  • Sign into your TurboTax account and open or continue your return. 
  • Select Tax Tools in the left menu. 
  • You'll find Tools beneath Tax Tools.
  • Select View Tax Summary
  • Select Preview My 1040 (left black panel)

There are also some articles that may be helpful:

Why did my refund go down compared to last year?

Why did my refund drop when I entered a second W2?