Basically, if ancestry happened to test a SNP that was an indicator of a deep haplogroup and you are positive for that, then lucky you. Otherwise, you might just get something very common like R1b, or none at all.
The best thing to do is look at the ISOGG information because the haplogroups are always changing and Promethease is using an out of date tree. The following link is to a list of the SNPs related to haplogroups and what they indicate. Promethease will tell you the SNP, and then you can look it up at ISOGG. http://www.isogg.org/tree/ISOGG_YDNA_SNP_Index.html
You can then look at the the haplotree to see where your haplogroup fits in.http://www.isogg.org/tree
You will also be a member of all the groups above the one you tested positive for.