Why does my SPICE simulation have a different lambda value from my model files lambda value?
I tried it myself using LTspice and am able to get the expected result.  It's possible your procedure is flawed, so I'll lay out how I typically do this measurement/calculation.

First, \$\lambda\$ (aka the &quot;channel length modulation parameter&quot;) is calculated using the output characteristics (\$I_{D}\$ vs. \$V_{DS}\$) when the MOSFET is fully in saturation.  At this point the output curve forms a straight line.  To get \$\lambda\$, you must first extrapolate this line and find the \$V_{DS}\$-intercept.  For different \$V_{GS}\$, this \$V_{DS}\$-intercept point will be the same, so \$\lambda\$ will also be the same.  The following image from this website highlights this behavior:

You can use the equation for lines to get an easier expression to work with, which I'll do below.
$$
\begin{align*}
y = mx + b &amp;\implies x\text{-intercept} = -\frac{b}{m} \\
&amp;\implies -\frac{1}{\lambda} = -\frac{I_{D}\text{-intercept}}{\text{slope}} \\
&amp;\implies \lambda = \frac{\text{slope}}{I_{D}\text{-intercept}} \\
\end{align*}
$$
I like to do one more simplification, which allows us to plug in values directly from the LTspice plots without having to explicitly calculate the \$I_{D}\$-intercept.  If we take a specific point on the line \$(V_{DS_1},I_{D_1})\$ we can rewrite as:
$$
\begin{align*}
&amp;I_{D}\text{-intercept} = I_{D_1} - \text{slope} \cdot V_{DS_1} \\ \\
&amp;\therefore \text{ } \lambda = \frac{\text{slope}}{I_{D_1} - \text{slope} \cdot V_{DS_1}}
\end{align*}
$$

With the above information, we can perform the simulation on the circuit in question.  I chose your same \$V_{GS}\$, but swept the \$V_{DS}\$ up through 20V because that's around where I was satisfied with &quot;straightness&quot; of the line.  When plotting \$I_{D}\$ vs. \$V_{DS}\$, I selected points at 20V and 19V to get the slope.

Using the slope and one of the points (I used the one on &quot;Cursor 2&quot;), we can solve the equation derived above:
$$
\lambda = \frac{\text{slope}}{I_{D_1} - \text{slope} \cdot V_{DS_1}} = \frac{(637.561 \times 10^{-6})}{(55.255 \times 10^{-3}) - [(637.561 \times 10^{-6}) \cdot 20]} = 15 \times 10^{-3}
$$