That fact can explain the differences between manual and automatic segmentations. 
But how can I explain the differences just for the left-hippocampus values in Freesurfer? Comparing percent volume difference between Freesurfer-Manual and FSL-Manual, the percent is bigger in left-hippocampus for Freesurfer-Manual. And according to the literature (Morey et al.) the percent volume difference for Freesurfer-Manual is always smaller than FSL-Manual as you can see in Fig. 5 of that article.

                        FSL                         Manual                        Freesurfer
HDHEHD HEHDHE
3810.7416983382.601803  3355.5 3007.537053721
3409.8294672860.1872242868 2653.533823331
       
3306.4098913381.885156 3025.53001.53358 3575
3794.767147 2907.55911628802769 36513474
4521.4508974587.515346 364234684049 4113
3794.638317 3841.11134830423066 40614242

     
4772.512671 4028.949924 35703496.54812 4801
        
4025.0371153833.11259 3772.535134094 4224
        
??3196.5 2704.5??
       
2900.8389382705.713765 252624663184 2820
3607.996048 3061.210132749.5 258938523591
       
3694.4012053343.946802 268225593530 3507
4562.195757 4484.2152513156 3286.541144599