This is an excellent question. Since we categorize cells into tissues based on similarities of both appearance and function, it stands to reason that all the cells of a certain tissue would look the same, and they do, to certain extent. However a closer examination would reveal that, despite the similarities among cells of a given tissue, no two cells are exactly alike.
This is true because of the many steps required to differentiate a cell. Every cell in an animal or plant body traces its ancestry back to a single, undifferentiated, fertilized egg cell. As that cell multiplies, its daughter cells begin to follow different developmental paths, which are guided by a variety of different factors.
Factors that help to drive cell differentiation include hormones, crowding, and surface attachment or lack thereof. Most cells do not function totally independently, and must create physical connections to other cells nearby, adjusting themselves to fit between their neighbors; this results in variability in size and shape. Due to their location some cells may have better access to nutrients, oxygen, or sunlight, and this could cause structural or size differences.
Over the past decade the studies of chaos theory and of fractal geometry have opened up new avenues of study, and many tissues in the human body are now known to show a fractal-based cell structure.