This is due to the fact that the second "𝐚" is located after an 𝐮𝐧𝐚𝐫𝐲 𝐩𝐥𝐮𝐬.
The unary plus 𝐞𝐱𝐩𝐞𝐜𝐭𝐬 to be followed by a 𝐧𝐮𝐦𝐛𝐞𝐫,
𝐛𝐮𝐭 here it is a 𝐬𝐭𝐫𝐢𝐧𝐠: "a". ( +"𝑎" 𝑔𝑖𝑣𝑒𝑠 𝑁𝑎𝑁)
That outputs a 𝐍𝐚𝐍 value which corresponds to 𝐍𝐨𝐭 𝐚 𝐍𝐮𝐦𝐛𝐞𝐫.
So we have: "𝒃𝒂" + 𝑵𝒂𝑵 + "𝒂" giving 𝐛𝐚𝐍𝐚𝐍𝐚.