The SPAM is getting past your ISP and is being filtered differently on your desktop/laptop/iPad.
On your iPad, make sure you mark the SPAM as SPAM before deleting it.(With the message open touch the flag icon at the top and select move to Junk) The iPad can learn, but you need to teach it. Can be tedious at first but it should improve quite quickly.
Edited to add: Apple actually has quite good SPAM filters with their own @icloud.com email addresses. One workaround would be to get an @icloud address and have all you yahoo emails forwarded to it. Then you'll have both yahoo and apple filtering your mail. A quick google suggests yahoo isn't that good at filtering SPAM (my impression only, I don't know one way or another). I use an apple address and I get only a couple of Spam messages a month.
Last edited by ExXB; 29th May 2016 at 08:08.