Off topic:Illumina NextSeq shorter fragment size than expected

Question

[BAGeno]

Hi,

We were preparing library for NextSeq whole exome sequencing. Our starting concentration before library preparation for three samples were 27ng/µL, 37ng/µL and 42ng/µL. We pooled library and when we checked our fragment size before running on sequencer by Bioanalyzer, we saw fragment size of 239 base pair which is smaller than expected. I want to know on which step we are making mistake which resulted in smaller insert size than expected?