convert-ly behaviour with chords

[Graham Percival]

I've found some interesting behaviour with chords:

----
\version "1.8.1"
\score{ \notes {
r1
<<a2 d>>
}}
----
address@hidden:~/tmp$ convert-ly foo.ly
convert-ly (GNU LilyPond) 1.9.7
Processing `tocc.ly' ... Applying conversions: 1.9.0, 1.9.1, 1.9.2, 1.9.3, 
1.9.4, 1.9.5, 1.9.6, 1.9.7, 
\version "1.9.7"
\score{ \notes {
r1
<<<a d>2>>
}}
----

The file does compile correctly, but why is the <a d>2 chord enclosed in << >> ?
Is this a deliberate decision?  I'm just wondering if it might be the result
of a harmless bug (which might not be harmless in all circumstances)

Cheers,
- Graham