Sigma Unit in Avellaneda -Stoikov market making model

[dingo9]

In Reserve Price Equation and Reserve Spread Equation, sigma ** 2 is used to measure market volatility risk, like

            r[n] = s[n] - q[n] * gamma * sigma ** 2 * (T - dt * n)
            r_spread = 2 / gamma * math.log(1 + gamma / k) + gamma * (sigma ** 2) * (T - t / float(M))

sigma*2 is given by preset in demo code to generate brownian process, and it can be calculate by std of brownian path.
As I do such experiment, I found interesting thing

import numpy as np
import brownian as bm

delta = 2  # The Wiener process parameter.
T = 10.0  # Total time.
N = 500  # Number of steps.
dt = T / N  # Time step size
m = 100  # Number of realizations to generate.
x = np.empty((m, N + 1))  # Create an empty array to store the realizations.
x[:, 0] = 1000  # Initial values of x.
bm.brownian(x[:, 0], N, dt, delta, out=x[:, 1:])

# %% check std and vol risk price
std = np.diff(x, axis=1).std(axis=1) / np.sqrt(dt)
# all sequence std close to delta
np.testing.assert_allclose(delta, std, rtol=0.1)
# so vol risk is gamma * sigma ** 2 = 4 * gamma, risk value / S0 is 4 * gamma / 1000 = 0.004 gamma

# %% round x to min float price 0.1, std value and risk value is not change
x_round = np.round(x, 1)
std_round = np.diff(x_round, axis=1).std(axis=1) / np.sqrt(dt)
np.testing.assert_allclose(delta, std_round, rtol=0.1)

# %% If price Unit changed, like previous unit is dollar/gram, to dollar/10gram, aka new price is 10 * x

x_10 = 10 * x_round
std_10 = np.diff(x_10, axis=1).std(axis=1) / np.sqrt(dt)
# std also times 10 times
np.testing.assert_allclose(delta * 10, std_10, rtol=0.1)
# so vol risk is gamma * sigma ** 2 = 400 * gamma, risk value / S0 is 400 * gamma / 10000 = 0.04 gamma

So, why vol risk changed when we just change price unit?