Unless the Blade of the Laer reshaped itself to be more appealing to Fulgrim's eye, based on the aesthetic of his legion, in order to entrap him? I could see a Slanneshi artifact being able to pull that off. Alternatively, it's one of those asynchronous things in HH, like how it seems that Horus was deploying the other legions to enable the Heresy before Davin happened (or I could just not be following their accounting of time very well). It's most likely FW went for aesthetics over logic though.