It is worth setting the tonal letters out together, because taken singly each looks like an anomaly and taken together they are a system. Gurmukhi has four voiced aspirate letters, one each in the velar, palatal, dental and labial series, and Ḍhaddha in the retroflex series makes five. In the older language each wrote a consonant.
In most modern Punjabi none of them does, and each instead signals that the syllable carries a tone, with the particular tone determined by whether the letter stands at the beginning of the word or later in it. Haha behaves comparably in a large class of words. Nothing else in the writing system marks tone, and no diacritic was ever introduced for it, so the whole of a genuinely phonemic feature of the language is recoverable only from a set of consonant letters that no longer name what they write.
Linguists describe Punjabi as the only major Indo-Aryan language to have developed lexical tone, and this group of letters is the written record of how it happened. The atlas records the older value and the modern one side by side for each letter rather than choosing between them, because both are current: the older reading survives in learned and liturgical recitation and the modern one is what is spoken.