Simply, 5 × 100 ＋ 5 × 11 ＝ 555.
The following is a somewhat strange ( or irregular ) interpretation.
In the case, it's interpreted that 5 is composed of five bars.
And, plural 5s are pasted together so as to fill a gap with an other bar.
Surely, an independent cross ( composed of bars ) can be found out in such a pattern in each side.
