Why does method to_bqm not maintain variables order?

Question

When converting to bqm with .compile().to_bqm(), the original order of the variables is not maintained. To reproduce:

from pyqubo import Array, Binary

n=3
m=12

X = Array.create('x', shape=(n,m), vartype='BINARY')

for i in range(n):
    a=0
    for j in range(m):
        a += 2.0 * X[i,j]

    diff = a - 1.5

    model = diff.compile()

    print("BEFORE: ", model.variables)

    print("AFTER: ", model.to_bqm().variables)

The first two printed lines are:

BEFORE:  ['x[0][0]', 'x[0][1]', 'x[0][2]', 'x[0][3]', 'x[0][4]', 'x[0][5]', 'x[0][6]', 'x[0][7]', 'x[0][8]', 'x[0][9]', 'x[0][10]', 'x[0][11]']
AFTER:  Variables(['x[0][6]', 'x[0][11]', 'x[0][10]', 'x[0][7]', 'x[0][4]', 'x[0][3]', 'x[0][2]', 'x[0][5]', 'x[0][8]', 'x[0][9]', 'x[0][1]', 'x[0][0]'])

How can I preserve the original order of the variables?