Could be. However, I just cited this to find a counterexample, not because I thought it was the likely explanation. I think it’s just because some HW 12+ years ago required vec4 alignment, at least in some circumstances. NV_parameter_buffer_object appears to have vestiges of this.
Related: What’s also interesting is that Vulkan GLSL has cast off this old limitation, now allowing you to use std430 packing on UBOs in Vulkan 1.2 (or VK_KHR_uniform_buffer_standard_layout).
So modern GPUs support this. And there’s no reason the vendors can’t offer this support for OpenGL GLSL as an extension, if they choose to.