  Recent angle-resolved photoemission electron spectroscopy (ARPES) experiments
demonstrate that the momentum dependence of the spectral gap in underdoped
cuprates does not follow a pure $d$-wave form [H. Anzai et a., Nat. Comm. {\bf
4}, 1815 (2013)]. This deviation is highly controversial. It has often been
interpretated as a proof of the non-superconducting origin of the antinodal gap
in the underdoped regime. In this article, we show that the measured angular
dependence of the spectral gap can be explained by the basic nature of pairs in
high-T$_c$ cuprates. Hole pairs, or {\it pairons}, form as a result of the
local antiferromagnetic environment on the scale $\xi_{AF}$, the magnetic
coherence length. The spatial extension of the pairon wavefunction beyond first
nearest neighbours gives rise to the anomalous angular dependence of the gap,
in quantitative agreement with experiments. This simple interpretation strongly
indicates a common origin of the nodal and antinodal gaps.
