Multiple jobs- married filling jointly

Do you mean why is more withholding taken out?  Because if you have more than 1 job or your spouse also works you have more total income which will be taxed at a higher rate.   And each job only withholds taxes like it was your only job for the year.  

The primary purpose of the W-4 is to withhold enough taxes so that there is no tax balance due at the end of the year.  On the 1040 tax return, each source of taxable income gets totaled up to calculate the appropriate tax.  So when there are multiple jobs, there is more income and potential for more taxes due.

Please note that each working spouse needs their own W-4 for each employer.

Ok thanks.