I am a singing teacher - also an operatic tenor with a 3 octave voice range
I would say that most untrained voices sing flat because they are using the throat to produce the sound and not the support - they clench or alter the throat position on difficult or high notes.

Morwenna I do not know how old you are but by using these methods and also placing the sound in the mask you can increase your range almost immediately by about 3 - 4 notes
Most sopranos start off with a comfortable A and increase the range with training and practice