I turn my S4 off at night, and on again in the morning.

Is there a reason why the highest number for 'BestShare' is always found within the first 45 minutes,
and then the following 12 or so hours it doesn't seem to go any higher? (it has been doing this for four days so far)

So for example,

one day I turn the machine on in the morning, first 45 minutes,
bestshare is found at 7561782, stays at that number for the next 12 hours or so.

another day I turn the machine on in the morning, first 45 minutes,
bestshare is found at 14561782, stays at that number the next 12 hours or so.

Now I understand that it will stay at that number if it doesn't find anything better,
but why is it always peaking during the first 45 minutes and thereafter nothing higher?

I am using solo.ckpool.org and I am just wondering if this is normal.