The underbrace and underbracket commands seems to remove the whitespace after follow up symbols.
In the following MWE the space between the + and c is too small:
\documentclass{scrbook}\usepackage{mathtools}\begin{document}\begin{equation} a = c \cdot \left(\Delta t_{a g_i} - \Delta t_{a g_{tr}}\right) + c \cdot \underbrace{\left( \delta_{g_i} - \delta_{g_{tr}} \right)}_{\mathclap{\text{Clock offset errors}}} + c \cdot \underbrace{\left(\delta_{a g_i} - \delta_{a g_{tr}}\right)}_{\text{Other errors}} \end{equation}\end{document}
It can be also reproduced with underbrace and also the mathclap doesn't seem to be the problem.
Only way I can think of is adding a \, between the second + and the c but I would still like to understand the underlying problem.
EDIT:It also doesn't work in this simplified version without using mathtools
\documentclass{scrbook}\usepackage{amsmath} \begin{document}\begin{equation} a = c \cdot (\Delta t_{a g_i} - \Delta t_{a g_{tr}}) + c \cdot \underbrace{( \delta_{g_i} - \delta_{g_{tr}} )}_{\text{Clock offset errors}} + c \cdot \underbrace{(\delta_{a g_i} - \delta_{a g_{tr}})}_{\text{Other errors}} \end{equation}\end{document}