It's technically not an F1, as both parents weren't from stabilized lines.
The blueberry pollen was from the F4 generation (orignal stock unique to DJ Short) and the female side was a select landrace Guererran circa '72 stock.
Landraces are heterogeneous populations of heterozygotes, making the female likely heterozyous for various traits.
As such, you'll find a small degree of variation within the line.
-Chimera