Derivative of a dependent variable

Hello,

I am using the values of a dependent variable 'phi' and its derivative in Matlab. For some reason, the numbers of nodes are different between the original dependent variable and its derivative.

For example,
for the original dependent variable

phi_PDE=mpheval(model,'phi','dataset','dset1','Edim','domain','Selection','sel1');

The result is
phi_PDE =

expr: {'phi'}
d1: [11x10067 double]
p: [2x10067 double]
t: [3x19732 int32]
ve: [10067x1 int32]
unit: {'1'}

For the derivative

mag_gradPhi_PDE=mpheval(model,'sqrt(phix^2+phiy^2)','dataset','dset1','Edim',domain,'Selection','sel1');

The result is
mag_gradPhi_PDE =

expr: {'sqrt(phix^2+phiy^2)'}
d1: [11x10135 double]
p: [2x10135 double]
t: [3x19732 int32]
ve: [10135x1 int32]
unit: {'1/m'}

Could anyone tell me the reason for this?

Thank you,
Best regards,