All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
We explained previously the different categories of martyrs and underlined the difference between the martyrs in this worldly life and the Hereafter (those killed in the battlefield while fighting against the disbelievers) and the martyrs in the Hereafter only (all other types of martyrs); please refer to Fataawa 15027 and 117529. The fact that the martyrs of the second category are granted the reward of martyrdom does not necessarily mean that both categories are equal in reward in all aspects.
Ibn Qudaamah said: "Indeed, the reward of the martyr who dies in the battlefield fighting against the disbelievers is greater (than the one killed in the battle against the Muslims who rebel against the Muslim ruler) and he is higher in rank. It has been reported that he will be allowed to intercede on behalf of seventy of his family. His (the latter's) virtue does not extend to such a person and the same legal determinations do not pertain to him, because an analogy is only drawn between a thing and its like." [Al-Mughni]
Al-Munaawi commented on the Hadeeth that reads: "He who sincerely asks Allaah for martyrdom, Allaah will grant him the rank of martyrs, even if he dies on his bed." [Muslim], saying: "This does not mean that the martyrs of both categories are equal in all aspects; rather, they share the reward in principle only, not in its completeness." Therefore, Al-Munaawi reiterated: "This does not mean that they (martyrs of the two categories) get equal rewards…. Even if they share the rank of martyrdom, the great deeds done by the martyrs who died in the battlefield (and their sacrifice) entail extra reward and a special closeness (to Allaah); and this is the grace of Allaah that He grants to whomever He wills." [Faydh Al-Qadeer]
An-Nawawi expressed this meaning with the partitive preposition (of) as he said: "If he sincerely asks for martyrdom, he will be given (some) 'of' the reward of martyrs (who died in the battlefield)."
Allaah Knows best.