I've created a depth texture in OpenGL (using C#) as follows:

// Create the framebuffer.
var framebuffer = 0u;

glGenFramebuffers(1, &framebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);

// Create the depth texture.
var depthTexture = 0u;

glGenTextures(1, &depthTexture);
glBindTexture(GL_TEXTURE_2D, depthTexture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT24, 800, 600, 0, GL_DEPTH_COMPONENT, GL_FLOAT, null);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, depthTexture, 0);

Later, I sample from the depth texture as follows:

float depth = texture(depthTexture, texCoords).r;

But even when no geometry has been rendered to that pixel, the depth value coming back is less than 1 (seems to be very slightly above 0.5). This is confusing to me since, per the documentation on glClearDepth, the default value is 1. Note that this is not a problem of linearizing depth since I'm attemping to compare depth directly (using the same near and far planes), not convert that depth back to world space.

Why is my depth texture sample returning <1 when no geometry has been rendered?