The reason you feel a greater contraction of both soleus and gastronemus when doing a standing raise over a calf press on a leg press machine is again due to tension on the calf....
Since the gastronemus crosses the knee joint, extension of that joint effects the 'feel' and even functionality of the calf muscles....
When doing a seated calf press on a leg press machine most people lock there knees out (usually because they are using too much weight) which tensions the gastronemus... With this tension on the gastronemus the soleus works a lot less...
Now in a standing calf raise most people do not lock there knees out with puts less tension on the gastronemus and enables the soleus to play more of a role....
Less tension on the gastronemus (via greater knee flexion) results in more soleus activation...
My top tip is to bend your legs while doing calf press on the leg press.... Not just slightly either, you can bend then a fair bit and use your hands on your thighs for support.... This will give you better soleus activation during the movement and actually a better peak contraction on the gastronemus I have found....
Ink'd, I'm not lecturing you lol.... Even if it seems that way.... Just sharing a bit of info related to your experience stated in the post in quoting

for everyone...