unfold and tremolo have different math

[james]

The only difference between these two is the unfold/tremolo, so why  
do they render differently?
\version "2.11.65"

\relative c' {
   \scaleDurations #'(2 . 3) {
      \repeat tremolo 9 <c e g>8 \repeat tremolo 3 <c e fis>8
      \repeat unfold 4 { \repeat tremolo 3 <c d fis>8 }
   }
}
\relative c' {
   \scaleDurations #'(2 . 3) {
      \repeat unfold 9 <c e g>8 \repeat unfold 3 <c e fis>8
      \repeat unfold 4 { \repeat unfold 3 <c d fis>8 }
   }
}