Solution
RFmx WLAN is a measurement personality that extends the capability of NI RF instrumentation for WLAN 802.11 signal generation and analysis. One of the measurement it includes is the Spectral Emission Mask (SEM) measurement.
The expected mask shape is indeed what the standard defines. However, the mask shape is composed of relative power limits. In parallel, the standard also defines absolute power limits. That absolute power limit is standard-dependent, offset-dependent, and channel bandwidth dependent. For 11a/g, the absolute power limit is -49 dBm @ 100 kHz of RBW (-39 dBm/MHz), and it only applies to the 30 MHz offset when using a channel bandwidth of 20 MHz.
The standard forces the masks to be adjusted to the absolute power limits if it ever conflicts with the defined absolute value. That's why we see the masks being re-shaped in such ways when it reaches the 30 MHz offset. If the power of the generated signal increases and the absolute power limit is respected, the mask shape will match what the standard defines.