I have said since many years ago that demo mode play is never the same as real mode play. Experienced players, like myself for one, can see the difference in the gameplay and the results obtained in demo play - they are always so much looser and much easier to get good wins!
Why? Maybe mainly because the actual RTPs of a game, any game, are not the same at any one time. The RTP in real mode is so much more condensed and heavier (more traffic) whilst in demo mode they are not.
No one can say for certain or know the actual reasons, but I do believe that the above is only one of the reasons.