Fuses being thermal devices, there is always a time lag. For standard fuses, it takes many minutes to blow at rated current. An example here is a Maxi fuse; it takes almost 2 minutes for a 20A rated one to blow with 30A amps current. So, for transient charging conditions, that probably explains why a lower rated one survives. You just can't get that much precision in a fuse....
http://www.littelfuse.com/~/media/automotive/catalogs/littelfuse_fuseology.pdf