Right, that is probably it. If you're interested, this is how Debian compares version numbers:
http://www.debian.org/doc/debian-policy/ch-controlfields.html#s-f-Version
Basically, it is compared lexicographically (dictionary order), except with numbers compared numerically, and ~ is special. In this case, it's simply the case that "1.1.5-2" is "newer" than "1.1.5-1...".
Note that the "-2" (anything after the hyphen) is NOT a new software version, but rather, a new package version of the same software. This means that if the fix has been put into the FluidSynth code trunk (and I'm not clear on whether it has), then it should automatically be included in 1.1.6. But in the mean-time, I wouldn't expect it to be in 1.1.5-2, because it means some Debian person has updated the package without including new software fixes, but inadvertently leap-frogged the version number over diwic's PPA version number. A temporary solution may be for diwic to bump his PPA version number to 1.1.5-2ubuntu1-diwicppa, or even 1.1.6~1ubuntu1-diwicppa (which will sort "newer" than all 1.1.5 packages, but "older" than 1.1.6 itself).